Gpslayer: New menu items to remove all tracks and waypoints in GPS folders.

Gpslayer: New menu items to remove all tracks and waypoints in GPS folders.

Data in Gps folders are likely short lived. Old data are often removed
before upload/download operation. The new menu items
(empty upload, empty download, empty all) are to make the task of
cleaning up the folders less painful (now that gps data are saved
and there is multiple selection is not yet implemented).

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>