Global shortcut to enter Eyedropper picker state

[brainwo]

It would be nice to use the Eyedropper tool on a temporary basis without switching away from the current tool, to the Eyedropper tool, and back.

[Keavon]

Blocked by #832

We need a way of entering a brief color picking mode which shows the color picker until you click to sample the color (or hit Escape/right click to cancel), rather than fully switching to the Eyedropper tool. This mode could be entered by pressing the global keyboard shortcut, or by clicking an eyedropper icon button next to a color somewhere in the UI.