Select correct index without timer

Summary:
    remove the timer that selects the index at startup
    that caused flickering and things moving around since
    index 0 got selected, then moved to the correct one only after a while.

    unfortunately there is not a place (a signal) that says
    "the view has been correctly instanced from the model" that
    would be where selecting the current index at startup would be the safest.
    a place that seems working correctly is on listview onCountChanged

    this has to introduce KCMLookandFeel::selectedPluginIndex
    that isn't very pretty, but the flickering seems to be gone and
    the kcm loads from the correct place of the view from the first
    frame is displayed

Test Plan: he flickering seems to be gone and the kcm loads from the correct place of the view from the first frame is displayed. Once this is in the final form, some other config dialogs need the same treatment: plasma theme and wallpapers

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3528