Snippets: Allow lowercase/titlecase/uppercase modifiers for variables

Use the same syntax already used in the custom wizard to denote
variables that are modified to be lower-/title-/uppercase:

 $tESt:u$ will become TEST
 $tESt:c$ will become TESt
 $tESt:l$ will become test

The snippet will be inserted without any name mangling happening.
Once the editing is done the name mangling is applied to all fields.

Change-Id: I7c1f5a1ad2bb5acf1b88b54de51bb39391c64763
Reviewed-by: David Schulz <david.schulz@digia.com>