VariableChooser: Rework

Allow multiple expanders to be registered for lineedits, e.g. a
local and the global ones, and actually show them.

Use a tree view in the chooser for somewhat more structured display.

Change-Id: I769f92144e5249f45e54381de52aa6973eb20118
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>