os: Add 'Framebuffer::Session::sync_sigh'

The new framebuffer-session function allows a client to register a
handler for receiving display-synchronization events.
os: Make buffer size of 'Reporter' configurable
os: Refine Attached_rom_dataspace::update
os: Add Rect::contains() to geometry utils
os: Add Point::operator ==
nitpicker: Consider menubar for pointer reports

The pointer-report facility used to report the screen-absolute position
of the mouse pointer. For nitpicker clients, however, this position is
meaningless because their coordinate is always constrained to the area
below the menu bar. This patch offsets the reported position
accordingly.