Fix focus shortcut when in the bezier curve editor

[mihe]

I was about to make a proposal for adding the ability to focus (by pressing the "F" key) on all the bezier keys in the bezier curve animation editor when no keys are selected, only to find that this seems to have been the intention all along, as seen here:

godot/editor/animation_bezier_editor.cpp

Lines 940 to 945 in 42e5b3a

	if (selection.is_empty()) {
	for (int i = 0; i < edit_points.size(); ++i) {
	IntPair key_pair = IntPair(edit_points[i].track, edit_points[i].key);
	focused_keys.insert(key_pair);
	}
	} else {

The problem was that when the view box for the focus was calculated, it was still relying on the selection set rather than the focused_keys set, which looks a lot like a bug.

This PR fixes that by simply using the focused_keys set for calculating the view box instead.

This would also explain the somewhat janky behavior when focusing while having only one key selected, which this focused_keys set takes into account by adding the neighboring keys to it. So I guess this PR fixes that too.

[akien-mga]

Thanks!