Re-submission of #1256 (app-system and menu-editor fixes) 

Resubmission of #1256:

copy-pasted - 

Menu editor:

Removes separator button
Removes move-up and move-down buttons for moving apps. You can use them to rearrange the categories.
Cinnamon:

remove app-system settings-tree functionality - functions return null or empty to maintain compatibility.
remove all calls to settings tree
merge cinnamon-settings.menu with cinnamon-applications.menu
Issues:

For existing installs, the Preferences and Admin (at least for me) showed up at the top of the list in the editor and the menu applet. Resetting to defaults in the editor puts them in alpha order, and the user can move them up or down the list as they desire.

Long term:

Need to decide if we want to allow re-ordering apps in the menu applet. This will require some significant changes, as currently the applet is optimized for speed, and alphabetizes the entire list during loading, then merely hides/shows items in each category. However, if we do this, we'll need to really fix the positionItem function in the MenuEditor.py. Right now is a bit of a hack, but it works for categories.