Doc: Small fixes

Change-Id: I163f69c14ecbc07a66b1bac7c5a53b22405c895e
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Expose hovered to our controls and document it consistently

The current solution is messy and we only expose it partially to
some of our styles. I think we should simply expose this to all.

Change-Id: Ia546d5657ea416df99c9d9d92cc714b7f1c928fe
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Replace backgroundColor with backgroundVisible

This exposes backgroundVisible to TableView and
replaces backgroundColor in TextArea.

We think the backgroundVisible is more flexible and allows us
to find a more appropriate solution to support palette in the
future.

Change-Id: Icfcd512c29cf030059c4ce2d32feb402d1a33662
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Fix missing headerview gap when not showing frame in TableView

Tested on all three platforms and looks better.

Change-Id: I8a68dfc7fe97a05c4313ac10de2f93dabec5a964
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>