xlib: Fixed small race condition in wait_for_x_event.
xlib: Simplified show/hide cursor.
xlib: Bashed noisy assert.
xlib: Locking updates.

- Bashed xlib lock. The documentation for XInitThreads says we don't need to
call xlib's lock and unlock functions if we protect access manually, which
we do.
- Added asserts for most functions in xlib-view that the view is not destroyed
and the lock is locked, where appropriate.
Bashed flashing cursor test.
Vec and Box no longer default initialize.

Box's default ctor was taking 78% of running time. It now takes 2.81%.