In order for FvwmEvent to know when certain RandR events have occured,
change the way RandR events are tracked, and set flags on the monitor to
indicate what RandR changes have happened.

Only debug logging tracks this, the changes for FvwmEvent will come
later.

This adds the work necessary to make FvwmEvent listen for RandR events.
Three events are supported:

* monitor_enabled -- when a monitor is "plugged" in, and active;
* monitor_disabled -- when a monitor is "unplugged" and deactivated;
* monitor_changed -- when an enabled monitor changes resolution/rotation

An "emit" flag is used to track events, separate from the on-going state
of RandR itself.

Fixes #26