Set wayland output scale

Summary:
Provides a virtual method in Screens where backends can supply the scale
of each screen, this is then set on each output.

For the X windowed backend this value is taken from a command line
parameter.

Test Plan:
Ran windowed mode with --scale 1 and 2
then kate --platform=wayland from another screen.
On the latter case UI elements were scaled up correctly

Reviewers: #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3159