x11: factor out normal window resize code

As preparation for resizing the window with input commands in the
following commit.

Since there are already so many functions which somehow resize the
window, add the word "highlevel" to the name of this new function.
command: add property to scale window size
command: add generic "multiply" command

Essentially works like "add".
command: replace speed_mult with multiply command

The compatibility layer still takes care of the old speed_mult command.