Make web-based widgets default

[linev]

Following widgets will be web-based by default:

1. TCanvas
2. TBrowser
3. TTreeViewer
4. TGeomPainter

Type of all these widgets can be configured via rootrc file:

# As options for Canvas.Name, ROOT provides TWebCanvas and the legacy TRootCanvas.
Canvas.Name:                TWebCanvas

# Type of Browser: TRootBrowser or TRootBrowserLite or ROOT::RWebBrowserImp
Browser.Name:                ROOT::RWebBrowserImp

# Type of geometry painter: root or web
GeomPainter.Name:            web

# Type of tree viewer: TTreeViewer or RTreeViewer
TreeViewer.Name:             RTreeViewer

Any of these settings can be adjusted in local .rootrc file.

One can provide special config for web-based fit panel.
For the moment TH1::FitPanel() always starts classical GUI and there is no configurable plugin for that.
The only problem - web-based fit panel was not tested for long time.

[github-actions]

Test Results

    13 files      13 suites   3d 0h 17m 0s ⏱️
 2 646 tests  2 645 ✅ 0 💤 1 ❌
32 580 runs  32 579 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 9bddf5d.

[dpiparo]

LGTM - NOT to be backported to 6.32.
Thanks @linev !