WIP: Ligature support

[bobbylight]

To render ligatures, but also support our "system selection" property where selected text is a different color, we need to update the token painting for partially-selected tokens. We need to render the token twice - once for the unselected piece(s) and once for the selected piece - to ensure ligatures are properly rendered in "both" pieces. Otherwise, if a ligature is partially selected, we'll render the individual chars instead.

Test ligatures to copy-paste:

// ligatures: ## <> => ++ ~~ <= >= <=> ->>

Things to consider:

• RSyntaxTextArea.setFont() should update all fonts in the SyntaxScheme with ligature-related attributes
• SyntaxSchemes should propagage ligature attributes to derived fonts
• Themes should propagate ligataure attributes to derived fonts
• Selections that begin or end mid-ligature should not render the individual glyphs (no line wrap)
• Selections that begin or end mid-ligature should not render the individual glyphs (with line wrap)
• Should there be an API for quickly setting this? - no, just document the Swing API and setFont() example

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.29703% with 30 lines in your changes missing coverage. Please review.

Project coverage is 74.51%. Comparing base (b230bf1) to head (309d017).

Files	Patch %	Lines
.../src/main/java/org/fife/ui/rtextarea/FontUtil.java	40.54%	17 Missing and 5 partials ⚠️
...org/fife/ui/rsyntaxtextarea/WrappedSyntaxView.java	61.53%	2 Missing and 3 partials ⚠️
...c/main/java/org/fife/ui/rsyntaxtextarea/Theme.java	71.42%	0 Missing and 2 partials ⚠️
...n/java/org/fife/ui/rsyntaxtextarea/SyntaxView.java	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #554      +/-   ##
============================================
+ Coverage     74.29%   74.51%   +0.21%     
- Complexity     6839     6871      +32     
============================================
  Files           177      178       +1     
  Lines         30269    30270       +1     
  Branches       3915     3920       +5     
============================================
+ Hits          22489    22556      +67     
+ Misses         5940     5889      -51     
+ Partials       1840     1825      -15     

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