Add airplane mode switch to popover; add middle-click tooltip

[jeremypw]

Fixes #6 Fixes #8

Needs input on:

• position of airplane mode switch
• whether the switch should be available in the greeter as well as in the session

[jeremypw]

Tooltip

Popover

[ryonakano]

I'm not familiar with UX, but I feel like the airplain switch in the popover should be placed in the top of it; the number of wireless networks is mutable, so the position of switch also may differ every time users opens the popover.

Also, the actions triggered by middle-clicking in other indicators also have a switch for that action in the top of their popovers, so placing the switch there would be consistent.

[jeremypw]

@ryonakano Thanks for the review! I can easily put the switch right at the top of the popover instead of right at the bottom. Anywhere else is a little more complicated.