project: Update viewer size when using set_video_properties

The width and height are set through set_video_properties when importing the first asset and when Ok-ing the Project Settings dialog. In these cases the viewer size was not being updated.

The fix consists of connecting to the "video-size-changed" signal instead of "rendering-settings-changed", as it seems more suitable. But set_video_properties should also be brought in line with the other methods using _set_video_restriction to call _emit_change..