i3bar: Realign tray clients on map/unmap notify

UnmapNotify events are interpreted by i3bar as an action taken by an
application to hide its tray window. Likewise, MapNotify events are
interpreted as an action taken by by an application to show its tray
window.

The actual cause of these events may be the application itself, or the
result of some action taken by i3bar itself at the request of the
application in the course of the XEmbed protocol.

We respond by adjusting the size of the tray window and realigning any
tray clients that remain. This will make room for the mapping window or
close the gap left by the unmapping window when the bar is redrawn.

fixes #1110
Remove flex/bison from common.mk since they aren't used anymore