toggle-synaptics: shorten message

Doesn't fit in normal-sized OSDs anymore.
cap: add -O for displaying OSD after invocation by shortcut

Only displayed for area screenshots, which have a somewhat nonstandard
shortcut (Fn+Camera).
toggle-synaptics: re-add 'on/off'
cap: fail on empty imgur output
cap: exit helper if parent dies
cap: keep $id up-to-date