Long Press to Clear Mission

Long-press on the Trash icon in the Editor screen
to clear the mission waypoints (with confirmation).

Issue: 699
Merge branch 'long_press_clear_mission' of https://github.com/kellyschrock/droidplanner into long_press_clear_mission

Conflicts:
	DroidPlanner/res/values/strings.xml
	DroidPlanner/src/org/droidplanner/activities/EditorActivity.java
	DroidPlanner/src/org/droidplanner/dialogs/YesNoDialog.java
	DroidPlanner/src/org/droidplanner/drone/variables/mission/Mission.java
	DroidPlanner/src/org/droidplanner/fragments/EditorToolsFragment.java