This commit adds support the following properties to theming:
Flags:
- IsTitleCenter: true if the title should be centered.
Metrics:
- BorderThickness: The border width.
- BorderRadius: The border corner radius.
Gutter -- a space left of the text, before the ruler -- is not a part of
the ruler, nor should it be treated as such. This commit implements
gutter handling in LibGUI::TextEditor as part of mild cleaning up of the
gutter handling (breakpoint icons) in HackStudio's Editor.
This commit also enables separate theming of the gutter.
