Test: multi selection in explorer

[isidorn]

Refs: #1023

• win @ramya-rao-a
• mac @RMacfarlane
• linux @roblourens

Complexity: 4

We have introduced multiple selection in the explorer. Open the explorer and verify:

• Selection grows as expected using cmd + click, shift + click, or shift + up/down
• All the context menu actions which make sense in the multiple selection case work as expected (delete, copy, copy path, open to the side, reveal in finder, find in folder...)
• All the above actions properly work when executed using keybindings
• If you have exactly 2 items selected you get a Compare Selected command in the context menu
• You can drag and drop multiple items within the explorer to move or copy
• You can drag and drop multiple items from the explorer to the editor area to open in a specific group
• Write an extension command which works on multiple selection and verify this works as expected (in the context you will first get the foused resource, and then the array of all the selected resrouces, more details here