Resolve WB-420 WHen yoking one layer to another, the map indices are reset to...

Resolve WB-420 WHen yoking one layer to another, the map indices are reset to 1:  There were two problems: (1) When checking for existing yoked overlay, those in surface montage were not checked.  (2) The selected map in an overlay has always been tracked using a 'Unique ID'.  Logic, executed during file reading,  that ensured each map in a file contained a unique 'unique id' was removed due to the execution time when a file contained many maps.  As a result, there are some files that use the same 'unique id' for more than one map and this caused problems with map selection.  So, the overlay system was changed to track the selected map using its index.  For loading of old scenes (that did not save the selected map index), the 'unique id' will be used to set the selected map.