BibTeX file chooser selects *.bib by default (instead of all file types)

[stlanger]

• we need to make sure that users do not accidentally choose non-BibTeX files as reference database. In the file chooser we should at least remove the "all files" option from the file chooser. If an error occurs users should be informed without stack traces
• we should implement a method to create a new reference database (*.bib file) for a project

[Joeran]

as clarification:

if you create a new project, you can select an existing bibtex file. however, the file chooser allows to choose any file extension (and this option is pre-selected). the better solution is to pre-select "*.bib (BibTeX file)" (but keep the "All Files" option)