x11: Fix x-display-process-events to return #f when all events read.

x11: Fix x-display-process-events to return #f when all events read.

The plugin's version of x_display_process_events will return any
keypress even though some do not translate into any input (e.g. when
it IsModifier).  X-display-process-events would return #f in that case
and preview-events think all events were read.  Now x-display-process-
events returns #t and preview-events disregards it, continuing to read
events.  Many callers must disregard these non-events.

Punt property-notify events in the previewer; do not queue them.