Bug 1000744: re-introduce a checked state for panel-menu buttons. r=MattN, a=lsblakk

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extra : transplant_source : %0B%FE%1F%94%E8t%86tT.%DCC3%BDh%10%B7%1F%80-
Bug 966723: exit Customize Mode on menu-button click, not mouse-down. r=mconley, feedback=dao, a=lsblakk

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extra : transplant_source : %DF%9F3%C5%A4%9D%8F%1Cqo%2B%8C%93%F5%22%DE%DAMu%27