Add -MP option to gcc command line (phony depends).

This stops the build from requiring manual intervention (delete .dep
files) after source files have been renamed.
Set base-size and resize-increment when window is resized.

I think this is more compliant. It fixes resize behaviour under
i3 window manager at least.
Make terminalGetDisplayName() return a reference.
Renames so as not to clash with Xlib (s/Window/Widget, etc).
And rename the files (window.{h,c}xx -> widget.{h,c}xx).