Fix bugs in sizing TableLayoutPanel (Xamarin bug 18638)

Consider spans and absolute sizes in computing PreferredSize for
TableLayoutPanel.
Also fix bug in allocating unused width to SizeType.Absolute columns.

Change-Id: Ic5065b83ced8195d1c5be39087f33b0d632fc9a6
Merge pull request #980 from StephenMcConnel/bug-18638

Fix bugs in sizing TableLayoutPanel (Xamarin bug 18638)