Migration to MVVM architecture

[rvost]

In PR #6 you mentioned a plan to move to MVVM architecture. I made a small sketch of how this can be done from the state of that PR:

Pseudo-UML on the left shows how PboExplorerWindow state can be split into sub-view-models. Most of the current code belongs to view models layer and can be reused now and probably refactored later.

Although adoption of MVVM doesn't require any libraries it would be beneficial to bring one to have INotifyPropertyChange and Relay (aka Delegate) commands implemented. I suggest MVVM Community Toolkit as it only contains the essentials and can be easily replaced if needed.

I've picked #6 as a starting point because there will be conflicting changes in PboExplorerWindow xaml and code-behinde, also some work of splitting PboExplorerWindow into subview already done in that PR.

If suggestions look good, I can do this migration while you working on BisUtils, but PR need to be merged first.