The new life of the new icon view

Summary

This is a blocker for the GTK 4 switch and a step towards GtkGridView adopting. #319 (closed)

I've refactored the seminal view-model-controller code design into something as close as possible to the GTK 4 list model-list widget paradigm, before completing its re-implementation of canvas view features.

It also removes the canvas view as a whole, as well as EelCanvas.

Commits sequence

The 36 commits of this monolithic MR follow this logical sequence:

1. Emphasize GListModel;
2. Refactor widget construction;
3. Tweak icon visuals to make them closer to the canvas view;
4. Update event handling (as this part of the code has been spared in !699 (merged) to avoid merge conflicts);
5. Implement missing feature to reach feature parity with canvas view (except DND and clipboard);
6. Remove canvas view and cleanup leftovers.

Drag-and-drop and clipboard would need to be reimplemented in GTK 4 anyway, so that's why they are skipped for now.

Known regressions

• Drag and drop doesn't work:
  • to do after GTK 4 switch, which brings an all-new DND API.
• Cut items are not marked as such:
  • to do after GTK 4 switch, which brings an all-new clipboard API;
• Performance is bad for directories with many files:
  • GtkGridView is going to solve this, but is only available in GTK 4, so we must switch first;
• After loading the directory, the grid view doesn't get keyboard focus:
  • it may be a GTK 3/GtkFlowBox bug; I've debugged it for a couple of hours to no avail, let's check again in GTK 4. -- Indeed, a GtkFlowBox-only thing. Solved after porting to GtkGridView.
• If the view doesn't fill all available height, the blank space at the bottom cannot be used to start a rubberband selection. -- It's still a bug in GtkGridView gtk#3462 (closed)