Import of files other than alignment files by Desktop->Import-> or drag'n'drop

The desktop background drop handler in 2.10.2 and before is hardwired to expect that any file dropped on to the background area, or loaded via any of the File menu's File/URL/Textbox options will yield a set of sequences which should be shown in a new alignment window.

However - this is not always the case, since the user may profitably want to load files containing:
- colourschemes
- groovy scripts
- substitution score models
- etc..