Adds preview scaling to the right-click context menu for the preview
pane.  This allows the ability to, for example, zoom the preview and
edit the preview 1:1 (canvas/base resolution).  This was a missing
parity feature.  Additionally, also allows scaling to the
"output/scaled" resolution the program is set to.

When the preview is in scale mode and is the focused widget, you can
hold space and drag with left click to change the zoomed position.

(Notes added by Jim)

Closes jp9000/obs-studio#687