Add "Insert Variable" to right-click menu

Closes #56.

Add an "Insert Variable" menu item to the right-click menu for cells that
have variables enabled.  This contains sub-menus for each defined variable
group in addition to one listing all current variables.

Utility functions InsertVariableMenuBuild and VarNameFromMenuTag handle
most of the work interfacing the Variable Manager to the menu.  MenuBuild
just added a single menu item that InsertVariableMenuBuild will look for.
MenuDo just calls VarNameFromMenuTag to interpret any tags starting with
"USERVAR_INDEX_" and places the correct variable name into that cell of
the table.
Merge remote-tracking branch 'origin/master' into Add-Variable-Menu
Merge remote-tracking branch 'origin/master' into Add-Variable-Menu