[sddm-theme] Fix initial focus after SDDM QQC2 Port
QQC2 StackView has a major behavioural difference
This as a general rule makes sense, especially if you're pushing a whole
focus scope onto the stackView, it would automatically select the right
element. However as our base was not a focus scope the
SessionManagementScreen instance itself got focus, taking it away from
the child password box.
LockScreenUi has to be changed so that it sets the initial focus the
same even though the stackview is on QQC1.
qmlscene in the sddm-theme directory with the dummydata setup
locked the screen regularly
Lots of gammaray
Reviewers: Plasma, ngraham
Reviewed By: ngraham
Subscribers: davidre, plasma-devel
Differential Revision: https://phabricator.kde.org/D25261