Support undecorated windows in `WindowConfig`

[timboudreau]

Rationale: Palette style windows are useful for components beyond menus and combo boxes. This patch adds support for undecorated windows in WindowConfig.

Use-case: I have a fairly large popup I need to show to select something where a lot of visual detail needs to be visible to the user; while I have this implemented with add_overlay(), I do not expect that to be usable in a pleasant way if the window size is smaller than what the popup requires (scrolling would be an ugly solution).

Built and tested on Mac OS and Gentoo Linux without problems.