Library editor: Show categories as a tree view

[ubruhin]

SUMMARY

From https://librepcb.discourse.group/t/increase-librepcb-usability/722:

Both UI categories (component and package) should be treeviews not listviews. It should be made visible that a category is not a root category, but a child category. As it is now it is too confusing (am I the only one?). I solved this (for me) by only creating root categories in my library: “Integrated Circuits - Analog”, “Integrated Circuits - Digital”, etc. but a treeview would probably make more sense in the long run.

SOLUTION

Not easy to fix since a library usually contains only parts of the category tree, not all of them. Let's consider a category "A" is in library "L1", and its child "B" is in library "L2". If L2 is not installed and you open L1 in the library editor, the parent category of B is not available so it's impossible to render it correctly.

Maybe for such cases a virtual "Unknown" category could be displayed instead, probably we need to make some experiments to see what works well.