Module system improvements: Container modules

[ric2016]

Continuing the discussion from #3783, I propose a simple enhancement in ModuleService which allows defining container modules consisting of multiple sub-modules. See PR #4794 for details.

This allows custom module implementors to provide multiple charts, tabs etc. within a single main custom module (folder). It also makes it easier for users to disable partial functionality of a custom module (currently, in a module providing e.g. a chart, a tab, and a block, the chart cannot be disabled separately easily).

More advanced functionality would be good (better overview of sub-modules in the control panel, automatic disabling of sub-modules etc.), but taking into account other ongoing major projects, I'd prefer to have this aspect implemented separately than a perfect solution (much) later.

[arbor95]

+1

[ric2016]

@fisharebest I'd appreciate any feedback on this issue, and the related pull request.

In particular because webtrees development is so slow at the moment, I think it would be very useful to have this functionality for custom modules, so that these can take over some of the requested features.