Now that fonts know their own weight, we no longer need highlighters to
tell us which font to use. Instead, they can just say "this should be
bold" and we'll find the right font ourselves.
This patch introduces the GUI::SyntaxHighlighter class, which can be
attached to a GUI::TextEditor to provide syntax highlighting.
The C++ syntax highlighting from HackStudio becomes a new class called
GUI::CppSyntaxHighlighter. This will make it possible to get C++ syntax
highlighting in any app that uses a GUI::TextEditor. :^)
Sidenote: It does feel a bit weird having a C++ lexer in a GUI toolkit
library, and we'll probably end up moving this out to a separate place
as this functionality grows larger.
