Parse 'Simple' entries, the right way

Fix code style, stop turning clang-format off

tests: Fix off-by-one error while iterating over model rows

Add FlatpakSimpleEntry parser, serializer and tests

Similar to FlatpakFilesystemsEntry.

FlatpakPermissionModel: Expose CanBeDisabled role for convenience

Now QML supports checkable status for Environment section too.

FlatpakPermissionModel: Parse 'simple' values groups while loading overrides

Fixes loading disabled simple boolean values (i.e. the ones with ! bang character prepended), because they can not be compared using == string operator or QList::contains method.

FlatpakPermissionModel: Port fetching list categories to readXdgListEntry

It does not use proper parsing yet though.

Add test for overriding with the same enabled state as in defaults

Move helpers definitions to the top

They gonna be useful in other function in this file later.

FlatpakPermissionModel: Port reading simple values to use the new parser

PolicyChoicesModel: Mark all constructors as explicit