Qt: Multi-state capture start icon.

Add ".on" versions of the x-capture-start icon along with corresponding
code in the StockIcon constructor to look for ".on" variants and add
them with the QIcon::On state.

Make the plain versions of x-capture-start blue to match the general
application icon. The goal is to make the toolbar and app icon fins blue
when Wireshark is "at rest" and green when it's capturing.

Change-Id: I31f4f9d910fc99c41d7c63bd9a722db1611760c7
Reviewed-on: https://code.wireshark.org/review/4225
Reviewed-by: Gerald Combs <gerald@wireshark.org>