Move "Configure Desktop" menu item to the top of the menu

This came up in today's VDG meeting. A user who right-clicks on the desktop is highly likely to want to change the wallpaper, but the menu item that does this is the last item in the menu, not the first. We can move it up to the top.

The "Open with Dolphin" item is likewise moved up too so that we don't have two single-item sections in a row.

Here's the result in conjunction with plasma-desktop!141 (merged):

There is no change to the context menu for desktop files and folders.

cc @teams/vdg