Introduce TableView::pressAndHold(int row) [signal]

[ChangeLog][TableView] Introduced a pressAndHold(int row) signal
that is emitted when the user presses and holds a row.

Change-Id: I823a7fd534fd6fca78831f00e7ecbb9f2118e006
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>