Update ActionSetDescriptor to use IEclipsePreferences

[deepika-u]

Updates ActionSetDescriptor to read and write its action-set visibility preference using IEclipsePreferences instead of the
old Preferences API.

Changes

• isInitiallyVisible(): now reads the preference through Platform.getPreferencesService() instead of the instance-scope node directly, so the lookup resolves through the standard scope search order rather than only checking one scope.

• setInitiallyVisible(): now writes the preference via InstanceScope.INSTANCE.getNode(...) and flushes the change explicitly, logging a failure if the flush doesn't succeed.

• Replaced WorkbenchPlugin.PI_WORKBENCH with PlatformUI.PLUGIN_ID in both preference methods and the constructor's error status. WorkbenchPlugin.PI_WORKBENCH is only an alias for PlatformUI.PLUGIN_ID, and WorkbenchPlugin already marks it with a TODO to be replaced by direct use of PlatformUI.PLUGIN_ID.

[github-actions]

Test Results

   858 files  ±0     858 suites  ±0   58m 18s ⏱️ + 7m 36s
 8 314 tests ±0   8 071 ✅ ±0  243 💤 ±0  0 ❌ ±0 
20 811 runs  ±0  20 141 ✅ ±0  670 💤 ±0  0 ❌ ±0 

Results for commit d2ad7a5. ± Comparison against base commit 72be953.

[deepika-u]

@merks : When you get some time, could you take a look at this please?