Viewlet Refactor

[sbatten]

This PR refs #85164

The goal of this PR is to refactor the Viewlet structure so that code useful to enhancing panels in the same way is separated from the Viewlet inheritance path.

This PR

• Introduces new classes PaneComposite, ViewPaneContainer. ViewPaneContainer encapsulates the SplitView and View management behavior currently unique to Viewlet in a standalone class. As a derived class of Composite, PaneComposite composes ViewPaneContainer allowing this enhancement behavior to be shared between Viewlet and Panel.
• Updates all existing derived Viewlets, (e.g. Explorer), to extend from ViewPaneContainer. The goal is to make ViewPaneContainer implementations agnostic of their Composite container (Panel or Viewlet).
• Implements very simple Viewlet classes for each existing Viewlet to instantiate the new ViewPaneContainer.
• Renames ViewletPane to ViewPane

[isidorn]

@sbatten the changes you outlined here make a lot of sense to me.
I did not review the code, since there are quite some changes and a lot of it is duplicated work per viewlet if I am not mistaken. Point being: looks good from a birds eye view! Also if you would like us to sync let me know, though I believe @sandy081 provided great guidance.

[sandy081]

@stevencl I went through the changes and they all looks good to me over all. I did a quick testing and found following

• Title of the view is not show in the title of the viewlet when there is only one view

• Drag n drop of views inside viewlet is not working

• Remote explorer is not showing the filter dropdown

Otherwise most of the features are working good. 👏

[isidorn]

@stevencl let me know if you would like me to test this out. Or even better create a test plan item and assign me to it.

[stevencl]

@sbatten - I think comments from @isidorn and @sandy081 above were for you instead of me... :-)

[isidorn]

Yes! Sorry for the ping Steven Clarke :)

[sbatten]

@sandy081 updated to fixes the issues you found.

This PR will be checked in when master opens after endgame testing. I'll keep in up to date with master as necessary. Let's use an endgame test plan item (I will create and assign) to harden.

[sbatten]

Bringing in from TPI issue so as not to keep an issue open for non-checked in code:
Debug view is not merged into viewlet header if it only view in debug viewlet #86056

That issue is also not specific to my branch. Since there are no more issue specific to my branch and this has gone through an endgame review, I will merge to avoid more conflicts. If you have additional feedback, we can add it straight to master.