Add multi selection by click and drag

[giacomoalbe]

Summary / How this PR fixes the problem?

This PR handles click and drag multi selection, both inside artboard and outside (on freeitems).

It does create a new mode and a new ViewLayer.

Inside test is performed on item's bound. This needs to be completely included inside the selection bound.
Partial bound inclusion yields no selection.

Steps to Test

• Create some freeitems or some items inside an artboard
• Click on an empty area on the canvas
• Drag to include the items you'd like to select

Screenshots

click_drag_multi_select.mp4

Known Issues / Things To Do

• I don't know if ViewLayers and Mode are coded in the right way, please enlight me :)

[Alecaddd]

There's a segfault whenever I create an artboard and then select and empty area.

One thing that would be great to have is a live indicator of which items will be included in the selection.
Something like an "hover effect" around the items included in the selection area.
It could be done in a follow up PR.

[giacomoalbe]

There's a segfault whenever I create an artboard and then select and empty area.

Ok I fixed the issue in the last commit.

One thing that would be great to have is a live indicator of which items will be included in the selection. Something like an "hover effect" around the items included in the selection area. It could be done in a follow up PR.

Yes, this would be nice. And I also think it should be done in another PR :)

This should be ready to be merged!