This patch removes the internal system menu from WindowServer and
replaces it with two IPC API's:
SetSystemMenu(menu_id)
SetSystemTheme(theme_path, theme_name)
These API's will allow us to complete the SystemMenu program and
move it entirely out of process.
Shuffle around some menu related code from window manager into menu
manager. This still is not perfect, and results in a little more of the
window manager to be publically exposed - but this is another step
towards better seperation of concerns between menu and window manager.
We also move the mouse_event handling into a new function in menu manager
as event handling was beginning to become a bit chunky.
