Draft: kirigami-filepicker: Display thumbnails

I'm opening this merge request early in order to gather feedback, in case you have better ideas / experience with handling thumbnails.

Currently missing:

• Targeted updating of rows (not sure if actually required)
• Handle HiDpi scaling
• Removing pixmaps from the m_thumbnails hash map again, otherwise we are memory leaking with extra steps
• Color of folder thumbnails does not match users accent color