Set FileDialog::nameFilters to an appropriate value

There is a TODO in the file dialog implementation:

+ // TODO: Find correct mapping from mimetypes to namefilter.
+ // nameFilters does filtering of file based on file extension and
+ // no clear mapping available from mimetypes to file extension, so allow
+ // all files to avoid any inconsistency as of now.
+ nameFilters: ["All files (*)"]

This can use net::GetExtensionsForMimeType. I don't think there's a Qt API for it (or, at least I couldn't find one).

Also, the descriptions "All files" and any additional ones (like "Images" or "Videos") need to be localized.