FR: Make main view focusable

[jesseduffield]

Is your feature request related to a problem? Please describe.
Currently the main view can be scrolled with pgup/pgdn but it would be good to use the arrow keys for scrolling that too

Describe the solution you'd like
We should be able to hit 'enter' on a selected item in a side panel and have the main view focused, where we can then naturally scroll the view with the up/down arrows (or equivalent vim keybindings)

Describe alternatives you've considered
none

Additional context
none

[glvr182]

I could do that when i work on the gui package rework

[OliverJAsh]

This would also be useful for searching the contents of a commit.

[stefanhaller]

I opened a new issue for this with a bit more discussion: #3988