[Documentation] Set size policy of providers combobox to AdjustToContents

Otherwise the Combobox is to small. If we call initialize() directly instead of
through a QueuedConnection AdjustToContentsOnFirstShow works if the Toolview was
closed on start. However if you restart kdevelop while the toolview is open
the combobox will be to small, too. So just use use AdjustToContents.