Refactor icon positioner saving and loading

The icon positioner saving and loading is handled by QML, which makes it a bit difficult to work with the logic in positioner CPP files.

This change does the following:

• Explicitly save only on user interactions
• Moved the save and load of config to positioner.cpp
  • Easier debugging of the saving and loading functions
  • Still uses the JSON style of save/loading like the old one
    • Technically should be compatible with the old config, which was also JSON.
• Makes positionerApplet accessible from FolderView qml
• Various checks for checking if screen is enabled or not to avoid doing unnecessary work when screen is off
• A lot of fixes to avoid any racing conditions
• Removed any timers
• Added various unit tests

Alternative to !2603 (closed)

BUG: 482875 BUG: 494874 BUG: 472137 BUG: 433867 BUG: 494412 BUG: 468619 BUG: 449069 BUG: 495047 BUG: 453314 FIXED-IN: 6.3.0