Allow traits UI-based dispatch even when TraitsUI not available

[corranwebster]

This checks to see if there is a ui dispatcher available for Traits when initializing the Pyface toolkit, and if not sets one.

This PR also includes a change that makes _FutureCall behave more like the TraitsUI _CallAfter for the qt toolkit when there is no delay for the callback. This will potentially allow _CallAfter to be removed from TraitsUI in the future.

Fixes #362

[codecov-io]

Codecov Report

Merging #363 into master will increase coverage by 0.02%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
+ Coverage    35.2%   35.22%   +0.02%     
==========================================
  Files         463      463              
  Lines       25807    25820      +13     
  Branches     3835     3838       +3     
==========================================
+ Hits         9085     9096      +11     
  Misses      16338    16338              
- Partials      384      386       +2

Impacted Files	Coverage Δ
pyface/ui/qt4/gui.py	86.51% <100%> (+0.8%)	⬆️
pyface/ui/wx/init.py	71.42% <75%> (+1.42%)	⬆️
pyface/ui/qt4/init.py	62.5% <75%> (+4.16%)	⬆️

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