Make Split View Panes Resizable

1) Makes Split View Panes Resizable
2) Updates to the image cache buster (final piece to fix #6356)

@redmunds 
This is probably something that should have been done when the code was made to work with things other than "panels" (hence the: `createdByWorkspaceManager` discreet).  

There are extensions which are listening to these events so we can't rename them outright. `MainViewManager` only needs to listen to `panelResizeUpdated` so that can be cleaned up.  That leaves one internal instance listening to a panel named event that isn't a `panel`.  

We could pass a flag in like we do for `createdByWorkspaceManager` to denote if the thing we're resizing is a view and just change the name of that to `viewResizeUpdate` or something.  But IMO, that is probably cleanup work that we could do in the `Opportunistic Cleanup` phase.

If you're OK with that then let's add it to that card.