Toggle button for changing priority

[stayradiated]

@consindo is currently implementing a toggle button to set the task priority. This is useful on tablets that do not have a right click button.

Just some suggestions:

• [x] Add a tooltip so people know what it is for.
• [x] Add a transition so it fades out when the task is collapsed
• [x] Add a transition to the bottom part of the priority bar on the left of the task.