This commit introduces color scheme support to Terminal. These are found
in `/res/terminal_colors` and the default color scheme can be set in
`~/.config/Terminal.ini`. Furthermore, a combo box is added for
setting the color scheme at runtime.
The previously used default color scheme has been added to
`/res/terminal-colors/Default.ini`.
To make the implementation more compatible with other color schemes,
`TerminalWidget` now supports overriding the default foreground and
background colors.
