SplitView: calculate implicit size

Set the implicit size of the SplitView based on the sizes of the
split items.

Change-Id: I775b938215216831d10cb50868e4c6733e157eeb
SplitView: update code to use 'clampedMinMax' function

This function was added in a previous patch, so lets reuse it
in already existing code.
SplitView: rename 'expanding' to 'fillWidth'

Rename the variables used in the code to ease maintainance.
SplitView: always respect min/max