CPPEditor: Rework preprocessor addition tooltip.

Change the preprocessor popup to a dialog.
Save the additional preprocessor directives to the session instead of the
.pro.user file.

Change-Id: I0d08c5684cfb21e822cde0a965c9cf14e5d6d47d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>