User settings are used instead of installer settings even when user defined path does not exist

User overriden paths for MCU packages (board SDKs, toolchains etc.) are always preferred over the paths written by the installer, even if the path does not exist. This prevents creating kits when upgrading to a new Qul version if the user has ever had to manually specify the path to a package.

Solutions proposal:
Use the user defined setting only if it exists, if not, use the path from the global settings.
When fixing a package manually by pressing the small reset symbol on the lineedit, and it works, use the path from the installer, and wipe the user setting for this package (if applied)
Still TBD what to do if invalid user paths are ignored during kit autogeneration. Do we display dialogs asking to remove user settings, simply remove user settings (with/without warning), or do nothing at all about user settings?