app: add signal GimpWindow::monitor_changed()

and emit it when the window moves between monitors or screens.
modules: make display-filter-lcms.c find the right monitor...

...when its GimpColorManaged's GdkWindow exists but is hidden, which
happens for all but the active tab of a single window with multiple
images open.
app: make sure GimpImageWindow notifies all its shells of monitor changes

Call gimp_color_managed_profile_changed() on all newly added shells,
and on all shells in GimpWindow::monitor_changed().
app: some formatting cleanup in GimpImageWindow