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.
A "Tray" is a sunken container area for widgets. The first intended
client of this style is the GUI::FilePicker's common locations frame.
Thanks to @nvella for coming up with the term "Tray" :^)
