refactor(#2826): multi instance view

[alex-courtis]

fixes #2826

View does not mutate options, hence no need to deep clone anything.

Preserved the tab states bits; they still function as there is still one and only one explorer.

Testing methodology:

1. Add an error("$callsite") at each call to a View method
2. Test to ensure error is raised
3. Remove error
4. Repeat test to ensure correctness

[alex-courtis]

Apologies @raaymax I abandoned trying to rebase #2864 , I applied the changes by hand. Notable:

• Did not do the deep clone of the options member; they are not mutated by view nor callers
• Used classic class structure

I'd be most grateful for your review; any problems you remember would be good to check.

[alex-courtis]

• Bug: tree does not refresh and becomes unusable following cd

[alex-courtis]

This is working, however it's not useful due to the extensive use of the globals.

Alternative approach: extract the global functions into a separate global module, to be addressed in the next milestone.

[alex-courtis]

Needs a re-think and alternative approach.

[alex-courtis]

Split instance Window and global View.

Dogfooding.

[alex-courtis]

Splitting out view was a step backwards.

Revert that and continue in #3153