Maestro - Composite Line Editor SizeContext Units UI

[crispinatime]

When adding a new line to a composite line (that't the "Use composite lines" part of the classic linear feature style) the "Size Context" dropdown shows 'Device space' - however, what is actually stored in the XML is <SizeContext>MappingUnits</SizeContext>.

This is very hard to spot when editing lines and, if left unedited results in apparently incorrect mapping and the legend does not render as expected.

The workaround, once identified, is to either edit the XML or to set the composite line to Mapping Units, save and close, then re-edit changing the units back to Device space. Both are non-intuitive to the general user.

[jng]

Fixed r6393