Implement more UI code

[jonwil]

• Fixes Backspace key does not remove text in text fields #1078

[codecov-commenter]

Codecov Report

Attention: 1055 lines in your changes are missing coverage. Please review.

Comparison is base (1260d86) 2.55% compared to head (d1e9fc9) 2.53%.

Files	Patch %	Lines
src/game/client/gui/gadget/gadgetlistbox.cpp	0.00%	424 Missing ⚠️
src/game/client/gui/gadget/gadgetpushbutton.cpp	0.00%	199 Missing ⚠️
src/game/client/gui/gadget/gadgettabcontrol.cpp	0.00%	136 Missing ⚠️
src/game/client/gui/gadget/gadgettextentry.cpp	0.00%	110 Missing ⚠️
src/game/client/gui/gadget/gadgetcombobox.cpp	0.00%	95 Missing ⚠️
src/game/client/gui/gadget/gadgetcheckbox.cpp	0.00%	73 Missing ⚠️
src/game/client/gui/gadget/gadgetradiobutton.cpp	0.00%	17 Missing ⚠️
src/game/common/system/unicodestring.cpp	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1089      +/-   ##
===========================================
- Coverage     2.55%    2.53%   -0.03%     
===========================================
  Files          949      949              
  Lines       109418   110292     +874     
  Branches     18667    18881     +214     
===========================================
  Hits          2799     2799              
- Misses      106215   107089     +874     
  Partials       404      404              

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[xezon]

First batch

[xezon]

Second batch.

[xezon]

Check Box works
Combo Box works
Combo List works
Static Text works
Progress Bar works
Button works
Tab Control not tested, what is this?
Text Entry works

[xezon]

Looks good. No crashes. UI gadgets work in principle.