osc: make tooltip_an default property
osc: make text squuezing layout dependent

Wether and when the text of a button should be squeezed when it
gets too long can now be configured in the layout:

lo.button.maxchars = <number>
nil = no squeezing (default)

If the button text has more than <maxchars> characters, it will
be squeezed to the estimated width of <maxchars>.