Position perference, ordering priority and menu merging

[Timoses]

Implements #3

Adds the following:

• position preference and ordering priority for both menus and menu items
• menu merging: Menus and containing submenus with the same name are merged together
  • merging only takes place when there is a filetype collision, i.e. the items being merged have at least one filetype in common. Otherwise the items are left intact.
  • An error is thrown if two items with the same name are to be merged but contain different commands.
• displaying items according to the specified position preference and ordering priority:
  • entries with the same position are ordered by their priority and displayed subsequently
  • subsequent entries may not meet their specified position preference in case more entries were assigned to previous positions than fit before reaching subsequent entrys' positions
  • in case positions are left blank empty entries are displayed
• Venu function venu#unregisterAll to unregister all menus.

[ozars]

Thanks! This looks quite useful.