panel/popover: Add grab support, improve placement on screen

The popover will now grab the cursor, ensuring it will be hidden
again should the user click outside of the popover. Keyboard grabs
are not yet implemented.
panel/applets: Update menu applet and window to new popover API
panel/popover: Ensure we disappear when we're supposed to

We now disappear when the user hits ESCAPE, or indeed when we lose focus,
as well as when an area outside of bounds is clicked.
panel/icontasklist: Respond to TOUCH_END "clicks"