X11 (xcb) support for QScreen-per-output and runtime changes

X11 (xcb) support for QScreen-per-output and runtime changes

QScreen has notifiers for its properties, but they were not being
emitted when one changes the resolution or arrangement of individual
outputs, e.g. via xrandr.  Also there should be one QScreen per
"output", e.g. laptop LCD + external monitor means 2 QScreens
which will be siblings, rather than just 1 QScreen to represent
the whole desktop.

Change-Id: Ia61bbc5e6a3506f813ab11f87c03d14cf7f4ce85
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>