Make F4 (edit properties) work for multi selection

[tznind]

Currently there are 2 ways to edit properties. You can right click something and access its 'Properties' submenu

Then there is F4 (which displays all properties and values). View/edit all properties (default key F4 or click the view title in the context menu)

This is the Window ShowEditProperties. It currently only works for a singular Design. We should expand it to also work for multi selection in same way Context menu does (thanks to #42)