Handle multiple keypresses in example

[LilithHafner]

Two sequential releases resulted in an error because the first set time0 to notihing and the second tried to use time0.

Before:

julia> You pressed key 97 which is 'a'.
repeating key 115
repeating key 100
You released key 97 after time 121 msec.
┌ Warning: Executing #51:
└ @ Gtk.GLib ~/.julia/packages/Gtk/OyCvN/src/GLib/signals.jl:106
ERROR: MethodError: no method matching -(┌ Warning: Executing #51:
└ @ Gtk.GLib ~/.julia/packages/Gtk/OyCvN/src/GLib/signals.jl:106
::ERROR: UInt32MethodError: , no method matching ::-Nothing()::
Closest candidates are:
  -(::Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}) at int.jl:85
  -(::T, ::T) where T<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8} at int.jl:86
  -(::Union{UInt16, UInt32, UInt64, UInt8}, ::BigInt) at gmp.jl:534
  ...UInt32, 
Stacktrace:
 [1] ::(::var"#51#52")Nothing()widget
Closest candidates are:
  -(::Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}) at int.jl:85
  -(::T, ::T) where T<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8} at int.jl:86
  -(::Union{UInt16, UInt32, UInt64, UInt8}, ::BigInt) at gmp.jl:534
  ...::
Stacktrace:GtkWindowLeaf
,  event[1]:: Gtk.GdkEventKey(::var"#51#52"))(
widget   @ ::MainGtkWindowLeaf , ./eventREPL[346]:3::Gtk.GdkEventKey
) 
[2]   @  Main(::Gtk.GLib.var"#90#92"{Ptr{Gtk.GLib.GValue}, UInt32, Ptr{Gtk.GLib.GValue}, Vector{Any}, Bool, var"#51#52", Ptr{Any}}) (./)REPL[346]:3
   @ 
Gtk.GLib  [2]~/.julia/packages/Gtk/OyCvN/src/GLib/ signals.jl:87(::Gtk.GLib.var"#90#92"{Ptr{Gtk.GLib.GValue}, UInt32, Ptr{Gtk.GLib.GValue}, Vector{Any}, Bool, var"#51#52", Ptr{Any}})(
) 
[3]   @  Gtk.GLibg_siginterruptible (~/.julia/packages/Gtk/OyCvN/src/GLib/fsignals.jl:87::Gtk.GLib.var"#90#92"
{Ptr{Gtk.GLib.GValue}, UInt32, Ptr{Gtk.GLib.GValue}, Vector{Any}, Bool, var"#51#52", Ptr{Any}} , [3]cb ::g_siginterruptibleAny()f
::   @ Gtk.GLib.var"#90#92"Gtk.GLib{Ptr{Gtk.GLib.GValue}, UInt32, Ptr{Gtk.GLib.GValue}, Vector{Any}, Bool, var"#51#52", Ptr{Any}} , ~/.julia/packages/Gtk/OyCvN/src/GLib/cbsignals.jl:216::Any
) 
[4]   @  Gtk.GLibGClosureMarshal (~/.julia/packages/Gtk/OyCvN/src/GLib/closurefsignals.jl:216::Ptr
{Nothing} , [4]return_value ::GClosureMarshalPtr({Gtk.GLib.GValue}closuref, ::n_param_valuesPtr::{Nothing}UInt32, , return_valueparam_values::::PtrPtr{Gtk.GLib.GValue}{Gtk.GLib.GValue}, , n_param_valuesinvocation_hint::::UInt32Ptr, {Nothing}param_values, ::marshal_dataPtr::{Gtk.GLib.GValue}Ptr, {Nothing}invocation_hint)::
Ptr   @ {Nothing}Gtk.GLib,  marshal_data~/.julia/packages/Gtk/OyCvN/src/GLib/::signals.jl:58Ptr{Nothing}
) 
[5]   @  Gtk.GLib(::Gtk.var"#253#254") (~/.julia/packages/Gtk/OyCvN/src/GLib/)signals.jl:58
   @ 
Gtk  [5]~/.julia/packages/Gtk/OyCvN/src/ events.jl:2(::Gtk.var"#253#254")(
) 
[6]   @  Gtkg_sigatom (~/.julia/packages/Gtk/OyCvN/src/fevents.jl:2::Any
) 
[6]   @  Gtk.GLibg_sigatom (~/.julia/packages/Gtk/OyCvN/src/GLib/fsignals.jl:176::Any
) 
[7]   @  Gtk.GLibgtk_main (~/.julia/packages/Gtk/OyCvN/src/GLib/)signals.jl:176
   @ 
Gtk  [7]~/.julia/packages/Gtk/OyCvN/src/ events.jl:1gtk_main(
)

   @ Gtk ~/.julia/packages/Gtk/OyCvN/src/events.jl:1

After:

You pressed key 97 which is 'a'.
You pressed key 115 which is 's'.
You pressed key 100 which is 'd'.
You released key 97 after time 109 msec.
You released key 115 after time 89 msec.
You released key 100 after time 89 msec.

[coveralls]

Pull Request Test Coverage Report for Build 2819040286

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.8%) to 62.876%

---

Totals
Change from base Build 2795089809:	-1.8%
Covered Lines:	1631
Relevant Lines:	2594

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2819040286

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+5.5%) to 70.193%

---

Totals
Change from base Build 2795089809:	5.5%
Covered Lines:	1785
Relevant Lines:	2543

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #658 (2b4faf4) into master (ea4e6ab) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #658   +/-   ##
=======================================
  Coverage   65.60%   65.60%           
=======================================
  Files          32       32           
  Lines        2721     2721           
=======================================
  Hits         1785     1785           
  Misses        936      936           

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