Add "Follow Selection" in the 3D editor by using Center Selection twice

When pressing the Focus Selection shortcut twice, you will begin following
the current selection. This also applies to selection changes.

The effect is undone by pressing the Focus Selection shortcut another
time, using the Focus Origin shortcut, or by panning/using freelook
on the 3D editor camera. (Orbiting or switching between perspective and
orthogonal does not undo the effect.)

Co-authored-by: Robert Yevdokimov <robert.yevdokimov@autStand.com>