Simple ListBox for Borders and Fills panels

[Alecaddd]

I'm implementing a new VirtualizingSimpleListBox which is a simplified version of the listbox used in the layers panel. This simplified version is not scrollable and is way simpler. There's still a lot that can be cleaned up in there, but this is a start to have a simple container with a factory method to reuse widgets.

These are all the things I did in this patch, in not particular order:

• Fills and Borders panels now use the simple listbox.
• If multiple items are selected, and more than 4 fills or borders are getting generated, just break out of the loop. In a later patch I will handle the "Show all" link, and also not recreating the widget if the color is the same, but rather having 1 widget to handle all identical fills/borders of currently selected items.
• Added the borders panel. For now it's identical to the fills panel, more work is needed.

[Alecaddd]

Merging this because YOLO. These widgets will improve drastically with GTK4, but as a starter these are good.