Add the possibility to use any picture as the contact lists' and chat window background

[sjors]

As one of the sub-tasks for the task "Improve eye candy for KMess", I'd like to see the ability to select a picture for use at the contact list background and the chat window background.

The image should not be used right away, it would impair the ability to read contacts' names etc. For example, a transparent white image should be applied upon it. Same for the contact window; it could be placed in the KHTML view or in the background of the complete window! For this, we should check for Qt support.