Consider motif border for floating geometry

Consider motif border for floating geometry

When calculating the geometry of a floating window with motif hints that
specify a border style, take into account that this window will have a
different border style when calculating its position with
floating_enable() when the window becomes managed.

A nice side effect of this is that users can override motif hints with
`new_float` config directives when they are specified other than
`normal`.

fixes #1270