Extend search feature to allow replace as well

[vkbo]

Search/replace now fully implemented, although only for currently edited document

This completes issue/feature #48

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3b34408). Click here to learn what that means.
The diff coverage is 52.47%.

@@            Coverage Diff            @@
##             master      #51   +/-   ##
=========================================
  Coverage          ?   74.52%           
=========================================
  Files             ?       30           
  Lines             ?     4930           
  Branches          ?        0           
=========================================
  Hits              ?     3674           
  Misses            ?     1256           
  Partials          ?        0

Impacted Files	Coverage Δ
nw/enum.py	100% <100%> (ø)
nw/gui/mainmenu.py	93.61% <100%> (ø)
nw/gui/winmain.py	69.34% <33.33%> (ø)
nw/gui/doceditor.py	62.53% <5.88%> (ø)
nw/gui/searchbar.py	73.8% <74.46%> (ø)

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