[Window List]Fix initial focus

Summary:
Now arrow key nav works when opening a panel applet via keyboard
shortcut.
Fixes the same problem as D15856

Test Plan:
Open the window list applet with a keyboard shortcut, move
around with the up and down arrows

Reviewers: #plasma, hein, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16053