"Create Document" dialog improvements

1) Guess the alphabet of the input sequence. Modify the value in the "Alphabet" combo box.
2) Specify a default document location. This value should take into account the documents that already exist (both when they're written to the hard drive or not). By default, the documents must be stored either in .UGENE_Data folder or in its subfolder (e.g. "Created documents").
3) Check "Save file immediately" by default.
4) Enlarge the dialog.