Reveal the dock from any display

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
Plank should be able be called on any display.

**Describe the solution you'd like**
If plank is set to be always visible then it should be visible on main display and external displays, if it is set to auto hide then by just moving pointer to the bottom plank should appear.

**Existing work**
macOS has some good example of how it should be handled.

**Describe alternatives you've considered**
If plank is not able to "know" in witch display is been called, then just mirror the behaviour, if set to auto hide, then when moving pointer to the bottom should appear on all the displays.

**Additional context**
Working on multiple display is great, so not having the dock is frustrating if what to quickly change app.