Move 'old' overview to a default use of Multi canvas support

[rduivenvoorde]

See some discussion in this email thread: https://lists.osgeo.org/pipermail/qgis-developer/2017-October/050132.html

Below a picture of a project with both the Overview Panel (bottom) and a Multimap renamed to 'Overview':

[rduivenvoorde]

Please add points which should be fixed before we have all functionality of the Overview Panel, from email thread:

• Remember we must also remove the ‘hide/show in overview’ menu actions for legend items. (Tim)
• Make the layer context menu entries work with multi canvas type overviews. (Nyall)
• Add the overview style navigation (it works the opposite to the canvas navigation). (Nyall)

My take would be that maybe there is a button in the (second) mapview which with one push creates the same functions as there is now in the Overview Panel?