LibGUI+HackStudio: Move syntax highlighting from HackStudio to LibGUI

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.

Libraries/LibGUI/Makefile

@@ -14,6 +14,8 @@ OBJS = \
     ColumnsView.o \
     ComboBox.o \
     Command.o \
+    CppLexer.o \
+    CppSyntaxHighlighter.o \
     Desktop.o \
     Dialog.o \
     DragOperation.o \
@@ -49,6 +51,7 @@ OBJS = \
     Splitter.o \
     StackWidget.o \
     StatusBar.o \
+    SyntaxHighlighter.o \
     TabWidget.o \
     TableView.o \
     TextBox.o \