iconexplorer: Port window layout to SplitView, improve Preview pane
Note: IconGrid does not need focus: true
binding because the focus
would be taken by the search field first. In any case, setting focus
on a top level component rather than at the place of usage is wrong.
The boundsBehavior binding is obsolete by Kirigami.WheelHandler in our qqc2-desktop-style ScrollView style override.
Preview pane needed a little bit of margins, so that the text in the FormLayout doesn't touch the pane's edges. And labels should fillWidth rather than limiting maximum size.