Reimplement inset-select-all in a generic way

Reimplement inset-select-all in a generic way

There are 3 possible actions (in order)
* select current cell
* select all calls of inset
* select the inset from outside (in the containing inset)

C-a is now bound to inset-select-all

C-M-a is bound to the global "select all". The
reason for this change is that selecting an inset
is a more common operation that selecting an entire
document.

This patch is the result of discussion on #7727.