lockscreen: Allow GtkEntry to initially grab the focus too

With !1154 (merged) it became apparent that we initally focus the letter 1 rather than the GtkEntry after creating the lockscreen widget.

Hence make the GtkEntry sensitive so it can be initially focused (which matches the behaviour when one slides back to the overview and then back to the keypad). While at that slightly tweak readability around that code, see individual commits for details.

/cc @dos as you've been fighting focus behavior on the lockscreen recently and it touches code for that.