MaintenanceTool: Don't make "Remove all components" the default option

The IntroductionPage currently defaults to "Remove all components" which is not the most useful option (especially since the user has to go through a few login pages before seeing the IntroductionPage).

Cause
The current default behaviour occurs because QInstaller::BinaryContent::MagicUninstallerMarker is always inserted into MaintenanceTool.dat. IntroductionPage uses this value to set the default option.

Suggestion
Make the default "Add or remove components" or "Update components" by writing QInstaller::BinaryContent::MagicPackageManagerMarker or QInstaller::BinaryContent::MagicUpdaterMarker into MaintenanceTool.dat instead.

If, at run-time, MaintenanceTool detects no valid and enabled repositories, then it can switch to "Remove all components" as the default.