Reworked API:
 - only one [ExportCLASS] attribute is now required to export something
 - Fallback Tab metadata is not longer specified in the Tab but it is itself something constant in TabService
Improved fallback tab by providing a default tab with a default view
 - Prevents crashing of the program if no "default_tab" is specified
 - Does not allow plugin developers to redefine a fallback tab
Merge branch 'feature/api-rework' into develop