Designer property for "visible" now uses checkbox

Replaces pulldown that had text options. Fixes #172 

@halatmit @josmas 
Change-Id: I68a8a202dc24b5638defe9b7df2f3385a830e3c3
That will probably be fine. I'll try it tomorrow. The existing generic boolean property uses the same string value, so it should be good.

(originally I didn't use it because there's a comment that the underlying layer needs precisely the string "True", but turns out the generic one uses it too)