v2021.1.0

This is nearly a complete rewrite over many years, porting from Qt 3 to Qt 5. BrickLink also
changed a lot over the years, so a few things aren't supported right now, because I need to find
new ways to get access to the required data.

These are a few of the highlights, but I probably forgot a lot of things ;-)

Added

• LDraw support with 3D parts rendering.
• Lightning fast multi-threaded filtering and sorting.
• "Natural language" like filtering for the main document view (the tool-tip on the input field
  tells the whole story).
• Item and color id changes are now resolved automatically using BrickLink's change log.
• Modernized UI with Hi-Dpi support, still on all of Windows, macOS and Linux.
• Single-app mode: opening an BrickStore document from the Explorer/Finder will now just open a new
  tab in an already running BrickStore process.
• The font and icon sizes can be scaled between 50% and 200%.
• Undo/redo actions are now presented in a list with human readable labels.

Removed

• Custom UIs in printing scripts do not work anymore, because the JavaScript runtime is totally
  different. Even the normal printing scripts have to be adapted slightly, but in the end, I will
  have to replace the current mechanism once again for future proof Qt 6 based solution next year.
• Per-document column configurations and the "Collector" as well as the "Simple" view mode
  are not (re)implemented yet.