Systray: stack above instead of raising.

Raising the systray window puts it above a fullscreen window. Stacking it above the
statusbar doesn't.
Systray: don't force size upon resize requests.

The resize request events are now ignored. Some clients (deadbeef, pidgin) refuse to work
well with sizes different to the one requested. We try to follow the size hints whenever
possible, but if the client try to set an specific size we let it. If the icon gets too
large it will be naturally "clipped" by bh and the next icon to the right (which is raised
after the current one). A new configuration option prevents the bar from getting to small
for systray icons.
Systray: remove unnecessary events & checks.

Removed activate, deactivate, focus, modalize events.
Some code cleanup: unneeded guards.
Systray: remove all map/unmap/state stuff.

Is really needed? Clients seem to request dock and destroy, but not to map/unmap
in-between.
Systray: simplification, cleanup.