Now also packaged as Flatpak! Changes:
- support saving last index dir path
- fixed recent file list and made it also list directories
- fixed broken style of error banner (it had unreadable text)
- fixed blurry window icon on Linux
- support for Flatpak build on Linux (with fixes around its filesystem sandboxing)
- minor stylistic corrections here and there
- add MIME types to allow opening .pkg files or their folders in the app (on Linux)
- update to Qt 6.10 framework