Add button widget to Linux build.
Use screen-space coords in mouse events.

It's up to each widget to translate the screen-space coords to per-child
UI-space coords, since each widget needs to take into account the layout
flags.
Set the UI origin for each child in mouse events.

It was a little weird that the UI origin in each widget was the
*parent's* origin, so now we make it the child's origin.
Implement click events for buttons.