TableView: Raise focused and selected rows

Generally, styles give more emphasis to focused and selected rows in tables.
To help implement such styles, we raise those rows as follow. Focused rows
get a z-value of 0.7, selected rows, 0.5, and the rest a z-value of 0 (which
is the default).

[ChangeLog][TableView] Raise focus and selected rows above other rows

Change-Id: I215249781466d21c28de3cd1805b6d08d83ac856
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>