Improved code as suggested by Aurélien

This commit includes code improvements as suggested by Aurélien -
http://lists.hohndel.org/pipermail/subsurface/2014-March/010705.html

Changes made:
1. Code improvement suggestions incorporated
2. Marker added to the map which appears on long press
3. The marker also shows latitude and longitude on click
3. Select/Unselect all feature added in GPX import

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11 at iitbhu.ac.in>