Improve display and background related desktop context menu entries

[bitigchi]

Currently, there are two shortcomings in the Desktop context menu.

• Changing backgrounds is only possible through the add-ons menu, which is not immediately visible.
• It is not possible to change the resolution from the context menu.

These entries should be immediately visible through the context menu, and they should be labeled accordingly, e.g. not "Backgrounds" or "Display", but "Change Desktop background" and "Change display resolution".

Ideally, "Change display resolution" should not be a necessity, Haiku should immediately get the optimal display resolution, but still many people use Haiku in a VM setting, so eventually it is one of the first things they look to do.