Added GitDiffWidget and GitStatusWidget. Added m_GetLineHighlighting to TextFieldWidget.

Moved saving of TextFileWidget to its own OnChange(), to be called by
the contained TextFieldWidget.
Changed DiffWidget to use --unified similarly to git diff, along with
line highlighting.
ConnectionWidget<T> now goes through all widgets under pointer (until
first successful match), rather than Hoverer only.
Added DrawBoxBorderless.
Removed old class forward declarations.