Add dock placement settings in schema.
Add dock placement in pref.js.
Reload extension when positioning changes.
DashSlideContainer: implement sliding in all directions.
DockedDash: manage positining in all direction.

Merge _updateYposition and _resetPosition. Now staticBox use the actor position
and not vice-versa as before.
myDash: myDash actor handle multiple dock orientations.
myDash: dash handle different orientations.
PressureBarriers follow dock position.
Add position class to the dockedDashContainer.

.left, .right, .top, .bottom are added depending on the dock position.
myDash: correctly handle drag and drop horizontally too.