Implement MRU cache

[paulstn]

Switching to tabs when the order of tabs change and/or number of tabs change needs to be fixed.
Solution: Use a comprehensive cache that is a stack of tab ids - ids are unique (handles moving tabs) and the stack will present whatever tab is on top, if tabs need to be removed from the stack (handles deleted tabs). Will need logic to figure out where a tab's index is, or maybe even later cache indices for better performance.