(bug 43304) "change" event and snakType function for SnakTypeSelector

SnakTypeSelector now has a "snakType" function rather than "selectSnakType". This allows for
setting and getting the current Snak type. Whenever the selected Snak type changes, the "change"
event will be triggered.

Change-Id: I2411ce2cfcb3c2aa6eaf3ae2b2963dbfdd3039fa
Merge "(bug 43304) "change" event and snakType function for SnakTypeSelector"