The click handler of MainViewController was registered as a global handler, so
it was used when a side panel was focused that doesn't have a
SwitchToFocusedMainViewController attached (e.g. Status, Worktrees, or
Submodules). This handler would then push the main view context, but with the
code that is meant only for toggling between the main view pair contexts, i.e.
with taking over the parentContext from the otherContext, which doesn't have one
at that point. This would later lead to a crash in onClick because the
parentContext was nil.
Fix this by splitting the click handler in two, one for when it already has the
focus, and one for toggling from the other view, and make these focus specific.
In this commit this is only possible by pressing '0' in a side panel; we'll add
mouse clicking later in the branch.
Also, you can't really do anything in the focused view except press escape to
leave it again. We'll add some more functionality in a following commit.
