disallow loading of unknown and/or binary files

[szabgab]

We certainly should not try to open and display a binary file as we don't know how to do that.

There is a %EXT_BINARY in Padre::Document which is not in use that was probably designated to notice such files.

What about unknown file types? Currently we treat them as text files while we could probably just reject them and let the users know how to patch Padre...