Cleanup button handling in InsetInclude

Cleanup button handling in InsetInclude

The include inset has its own button_ object, that hides the button_
object of the InsetCommand parent. Remove that.

Remove the hand-made caching that uses the set_label_ boolean.

Let metrics() and draw() use the inherited methods. This requires to
use setBroken as needed and to implement inheritFont.

The use of InsetCommand::draw is what restores the hover effect.

Fixes bug #10415.