Deletes work when only one element is left
Stop drawing when mouse is up

Added another constructor for charge element. Now 2 constructors are for
initializing a line charge element by start point/angle/length and by
start point/end point and 1 for initializing a point charge. Slowly but
surely implementing functionality for line charge elements. MAJOR
PERFORMANCE ENHANCEMENTS: application now stops continuously drawing
whenever mouse is not clicked. Will be much less processor intensive
when app is idle. Didn't know why I didn't implement this earlier -_-