[ntuple] Initial integration with TBrowser

[jblomer]

Adds minimal browsing capability for RNTuple with TBrowser (browsing already works with RBrowser). Introduces the more general capability for foreign objects (not inherited from TObject) to implement custom browsing. This is done similar to a custom merge function, by a special method signature that is picked up by the dictionary generation and registered in TClass. This capability is then used by the RNTuple anchor object.

As a result, TBrowser can show the structure of an RNTuple. Note that RNTuple only uses the abstract TBrowser interface, which is part of the core library.

In a follow-up PR,histogramming data is to be implemented. The idea is that the graphical browser will register an RNTuple visitor for the drawing, so that the GUI will depend on the RNTuple library but not vice versa.

Icons in the tree view panel of the browser need to be fixed, too.

[github-actions[bot]]

Test Results

    20 files      20 suites   3d 9h 41m 18s ⏱️  3 090 tests  3 090 ✅ 0 💤 0 ❌ 60 205 runs  60 205 ✅ 0 💤 0 ❌

Results for commit 04fb0d43.

:recycle: This comment has been updated with latest results.