Disable file extension check if a custom bconf is provided

translate5 knows, which file extensions it supports. Other files are ignored in the import.

The okapi plugin adds all file extensions to this config, that okapi supports by default.

Yet it is possible to use filters like the regex filter in okapi to support other file extensions in certain rare cases, like .js for example.

The correct way would be, to parse the extensions-mapping.txt file of okapi, that is created when a bconf file is extracted. Yet since bconf is some kind of special okapi file, it is not possible to extract it by usual means.

Therefore to have a mid-term solution for translate5 until translate5 writes its own bconf-files for okapi and anyhow has to solve the problem of packing and unpacking bconf, we enable all file types to be passed to okapi if a custom bconf is provided.

That implies, that we change the behaviour that translate5 does not just ignore unknown files, but lists them in the eventlog. Same for files transferred to Okapi which could not be imported by Okapi.