[ntuplebrowse] Add treemap visualization in classic ntuplebrowser

[magnustymoteus]

This Pull request:

Changes or fixes:

• Add the icon for RNTuple-visualization node in config/mimes.unix.in and config/mimes.win32.in
• Visualization node represented by RVisualizationBrowsable class, which visualizes the corresponding ntuple on click as a treemap
• Use RTreeMap classes from included ROOTTreeMap module for treemap visualization
• Adapting the ntuple_browse test to the changes

Checklist:

• tested changes locally
• updated the docs (if necessary)

[magnustymoteus]

It seems like there is a single test failing from the changes, let me check into that

[magnustymoteus]

The test assumed that the first child of rntuple node in the browser was a field, which would now be the visualization node. I changed the test to reflect my changes and now it should pass.

[github-actions]

Test Results

    20 files      20 suites   3d 13h 47m 15s ⏱️
 3 663 tests  3 651 ✅   0 💤 12 ❌
71 547 runs  71 346 ✅ 189 💤 12 ❌

For more details on these failures, see this check.

Results for commit b3d6b06.