Install extensions though drag&drop on the extensions viewlet

- register viewlets and panels for containers
- events when view visibility has changed
- small refactorings

* wip 2

* use viewlet stubs

* fix layering issue

* fix saveState and contextmenus

* move context menu logic into panecomposite

* revert tasks.json

* tasks again

* add showHeaderInTitleWhenSingleView values

* patch tests

* some cleanup

* more cleanup

* start view

* merging conflicts

* fix title and dnd

* adding missing api surface to pane composite

* additional merge resolution

* lcd - revert layer promotion for now

* Revert "Fix #86347. Fixed overflow widget only for extension viewlet"

This reverts commit beb8a0c7.

* Revert "Re #85313. Fix suggest widget position for extension editor."

This reverts commit 9d471918.

* repatch titlebar for AA fix

* rename splitview panels to panes

* rename files

* customViewPane