changes #1

highlight polygon, go back to full extent when click a new layer, move
the zoom previous/next button
change #2

mouse wheel to zoom in/out, show pointer when clicking subtitle, move
zoom previous/next button to the top of the window, put checkbox in
front of each subtitle
Reconciled merge conflicts.

(DVA)
Merge pull request #16 from Georgia-Tech-Center-for-GIS/GretaYi-master

Reconciled merge conflicts.