The sidebar needed lots of intrinsec logic for working properly.
This fixes the problem of a huge sidebar when the environment is clean (aka. "new user" test).

This also fixes some problems with layouts. Now the sidebar will automagically adopt the needed size, and the only thing one can do with the slider is to hide or show it.

I have double reviewed it is correct with the kde pim coding style, but if something is wrong, please tell me.