Mail: change look of disabled text controls.

* Added HeaderTextControl that draws the text in black, and uses the
panel background without a frame when it's disabled. Only the label
is still drawn as disabled.
* Changed AddressTextControl to behave in the same way.
* The date view is now a HeaderTextControl, too.
* Unfortunately, the label is not vertically aligned with the contents.

Mail: work in progress to use the layout API.

Mail: change look of disabled text controls.

* Added HeaderTextControl that draws the text in black, and uses the
panel background without a frame when it's disabled. Only the label
is still drawn as disabled.
* Changed AddressTextControl to behave in the same way.
* The date view is now a HeaderTextControl, too.
* Unfortunately, the label is not vertically aligned with the contents.

Mail: work in progress to use the layout API.