Cached QHelpEngine files prevent installation of new releases

The QHelpEngine class displays online help generated by the qhelpgenerator tool.

Unexpectedly, however, the QHelpEngine class is caching help files which are left as artefacts after a program and associated help files have been removed, making it problematic when attempting to install newer versions of a program.

To reproduce:
1. Create a help file with one or more topics specifying one or more keywords.
Install the program along with the help files. Run the program and open help. The topics and keywords should be visible in the QHelpContentWidget and QHelpIndexWidget widgets.

2. Completely remove the folder containing the application and help files.

3. Add one or more topics and keywords to the help file.

4. Install the program along with the updated help files. Run the program and open help. The NEW topics and NEW keywords do not appear in the QHelpContentWidget and QHelpIndexWidget widgets. The list of topics and keywords is identical to what was shown for the previous version.

This makes it impossible to install a newer release of an application along with updated help files on a user's computer.

On a related note, the QHelpEngine class documentation up to and including the 6.5 Qt release is completely inadequate, it is a shame that this excellent tool is hampered by such poor-quality documentation.