New Advanced layout

[ferdnyc]

See #2703. The 2.4.4 layout is just nonsense.

This PR applies the layout proposed in my comment there:

What do people think of this? Obviously it's a little comical at the default window size, because the default window size is FAR too small for the OpenShot interface in its current form, especially if 2/3 of it is going to be dominated by the video preview. But with the window made larger, I think it'd be fairly usable.

(I really don't like Properties on the right. It puts the value on the outside, farther away from the Timeline, with the label in between them. That's just bad ergonomics.)

It also removes window_splitter and window_splitter2 from the settings file, as neither of them is used for anything.

[jonoomph]

Well, I like it! We will soon be revisiting the layouts in the next version of OpenShot, so I like the idea of improving ergonomics in the meantime.