Error caused by any file named as current theme and placed to directory
which is used for starting Qt Creator (formely user home directory).
For example, create empty file with name 'default', place it to your
home directory (for example: /home/user) and run Qt Creator. As result
you will see black corrupted screen.

To fix this bug I add checks of theme name. Now Qt Creator will only
load theme from current directory if name is explicitly provided by
the user or it is an absolute path.

Change-Id: I0255c8d220d84bd5b51de7ef9d64c66f7f57959c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>