WelcomeScreen: Pass height of develop mode to outer element

Previously the recent projects had an extra scrollview and the sessions
had a fixed height. In addition, as there will never be any scrollbars
around the recent projects anymore we don't need the borders around them
anymore, either.

Task-number: QTCREATORBUG-10731
Change-Id: I491265148a3ce777f15b8ba7d852248b583f2c83
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>