Implement auto-hidden menus

[jorisvddonk]

preferences -> auto-hidden menus / menus always onscreen (toggles between these two)

This feature, when enabled, automatically hides the parts of menus that you're not looking at.

It's hard to explain, but here's what's shown when you are looking at 'set remote target' from the Navigation menu:

(note that the complete left sidebar, as well as the lines of text, are hidden)
And here's what is shown when you then pitch down a little to look at the lines of text:

similarly, when looking at the main FCD menu on the left:

I don't see how this is a useful feature (I suspect it was implemented because text rendering in NIV is slow and a major cause of FPS loss in space), but I suppose it might be worth implementing this.