Label: Avoid changing alignment on linecount changes

This was the cause of one of the binding loop warnings in
DefaultToolTip. If text is wrapped and constrained it would change the
alignment which would can cause size hints tochange.

This code dates from before layouts. Modern code would be in a layout
that would align centre align the label within the layout rather than
position within the label. It also comes from a time where implicitHeight 
was also based on the lineCount, something we removed years ago.

Leaving it to the caller to handle alignment seems safer than having text
potentially jump based on the string length.

This is a behavioural change, and needs some QA though I haven't seen any issues.