Implement API extension for plug-ins needing information about vector chart objects.

Supports S-57/CM93 objects with OBJNAM attribute and _texto/_texta in CM93 (as this feature is widely misused for important chart objects there)
I did not implement any support for S-63 or or other vector charts provided by plug-ins. Also the API could perhaps provide more info - I did implement just what I need for https://github.com/nohal/objsearch_pi