For the Qt Creator variables, currently used in the external tools.
The variable chooser looks if the current focus widget has a variable
support property set and is a line edit, text edit or plain text edit.
For line edits it adds a little icon button that shows the chooser.
The chooser allows choosing variables from a list, and inserting them
into previously focused line edit, text edit, or plain text edit.
Variables are now registered with a description in the variable manager.
Also make the QT_INSTALL_BINS --> CurrentProject:QT_INSTALL_BINS for
consistency.
