Importer open dialog improvements

[stefan-kolb]

The open dialog of the importer import into...(new | current) db needs several improvements:

• the file type selection must filter the view based on suppported types
• the file type selection should display supported file endings for each type, e.g., medline|Pubmed? (*.nbib, ...)

[tobiasdiez]

Some of the backend code required for this improvement is available with #1207.

[Siedlerchr]

Regarding file filters, I recently added the code for this in the BrowseAction.
Have a look at this commit: #1304
The whole code for file opening will be finally reworked in #1336 (Atm I don't have time for that)