Locator: Shuffle the priorities a bit

Introduce a "Highest" priority for the filters-filter. Keep "Low"
available for lower priority than the default all projects and current
project filter.

Change-Id: I9152d2e6c64b65478416cee18183bce1240ee187
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>