View Container widget improvements

[olexii4]

We should improve the TreeView widget:

• Restore the previous widget state after reloading.
• Add each view container into the 'View' menu.
• Remove all unnecessary timers from the widget implementation. Add code clean up.

[akosyakov]

Please see #5665

It aims to bring state restoration of view containers and views.

Add each view container into the 'View' menu.

It will also provide a command to open a view otherwise there is no way to reopen a view container after closing it. Not sure about adding to View menu, it will pollute the menu fast. We can add view containers though instead there are not so many of them.

[akosyakov]

@olexii4 @evidolob Could you have a look at #5665? It:

• allows to toggle view containers contributed by the plugin system, so a user can reopens them
• should preserve plugin tree view layout state between user sessions

The PR is not final, there will be more changes to remove unnecessary changes, but feedback from the user perspective is welcomed.

[akosyakov]

In #5665 there is also Open View command now to activate any contributed view. And view quick palette prefix.

[akosyakov]

I believe everything was addressed in #5665