Fix Tab navigation in split editors (revert CTabFolder workaround, requires SWT fix)

[fedejeanne]

What

Fixes #4135.

This PR reverts TraversePageHandler back to its standard, original traversal behavior, removing the CTabFolder-specific special-casing that had been added as a workaround (loop-to-first/last-item logic, hidden-item detection, and the top-level CTabFolder parent-hierarchy lookup helper introduced by #2864 and the earlier attempt at fixing #4135).

That special-casing was only needed because CTabFolder itself did not loop over its own tabs when navigating with Ctrl+PageUp/PageDown, and it caused regressions such as bypassing the chevron drop-down list when tabs did not fit on screen (which is presumably the root cause of #4135, since the workaround interfered with focus traversal in split editors).

Dependency

This PR requires eclipse-platform/eclipse.platform.swt#3448 to be merged/available. That SWT PR fixes CTabFolder.onPageTraversal directly, so CTabFolder now correctly wraps around from the last tab to the first (and vice versa) on its own — without needing any workaround at the TraversePageHandler level, and without breaking the chevron behavior when not all tabs fit.

How to test

1. Make sure you have a build of SWT that includes CTabFolder: support circular page traversal in MRU mode without chevron eclipse.platform.swt#3448.
2. Open two editors side-by-side (split editors) so that Ctrl+PageUp/PageDown needs to traverse between the editor area's CTabFolders correctly.
3. Confirm that tab navigation with Ctrl+PageUp/PageDown behaves as expected and no longer misbehaves in the split-editor scenario described in 'Tab' navigatin Bug. Ctrl+PageUp -> Next Tab, Ctrl+PageDown -> Previous Tab. #4135.
4. Resize the editor area so that not all tabs fit and the chevron (drop-down list) appears; confirm that traversing past the last/first visible tab still opens the chevron drop-down as expected (this is the behavior that the old workaround used to bypass).

[github-actions]

Test Results

   855 files  ±0     855 suites  ±0   54m 41s ⏱️ + 1m 14s
 8 116 tests ±0   7 873 ✅ ±0  243 💤 ±0  0 ❌ ±0 
20 250 runs  ±0  19 596 ✅ ±0  654 💤 ±0  0 ❌ ±0 

Results for commit 5f1f875. ± Comparison against base commit 3f3daf7.

[fedejeanne]

I tested on Windows and it works as expected:

• Circular navigation (forward and backwards) is fine
• Navigation does not happen inside composed CTabFolders (e.g. in the Manifest Editor) but in the outermost level (it moves between classes, manifest editor and other views)
• If the chevron is visible, it opens it instead of navigating circularly
• There is no skipping an editor/view (as reported in #4135 (comment)).

Here's the video:

Video (Windows)

I couldn't test under Linux/Mac yet.

[HeikoKlare]

I've tested this together with eclipse-platform/eclipse.platform.swt#3448 on MacOS and there it works fine for me (with ordinary editor stacks, MRU stacks, with all tabs being shown as well as some in the chevron menu).