WIP: Ligature support. Selections ending mid-ligature is a problem

[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.

Current status:

• Seems to (mostly) work with the default SyntaxScheme!
• Currently testing with the demo app across all themes with the following text:

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

Current issues:

• Theme calls font.deriveFont() in a few places, and I think loses the base font's attributes
• Selections that start "in the middle" of a long token, such as an MLC, don't properly identify the start and end offsets of the selection when in the "Default (System selection)" Theme

Things to consider:

• [ ] RTextAreaBase.getDefaultFont() should support enabling ligatures
• [ ] 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
• [ ] For simplicity, should this be an API, e.g. textArea.setLigaturesEnabled(boolean)?