Add application state global to scene file

[nvkelso]

To support more interesting map styling, I propose adding the following set of Tangram global properties:

ux_application_state

• browse
• feature_selection
• search
• route_selection
• route_navigation
• route_preview (not yet implemented, could be used for Transit)

For instance, if the stylesheet knows we're in route_navigation application state then we can set all the text sizes to be larger, and reduce icon clutter on the map.

[sarahsnow1]

mapzen/android#372