Debugger: Improve initialization time

* Avoid QFileInfo where not needed
* Use a more accurate filter for QDir::entryList()
* Remove duplicate paths
* Set filters and filter types only once.

Reduces startup time by ~3%

Change-Id: I8896c08da5281e06672b7bdf6e8305ea394122a3
Reviewed-by: hjk <hjk121@nokiamail.com>