Bug 2090: Failure to handle old 1.3 documents with single-quoting style

[Helge Hafting]

Create a document with lyx 1.3  Set the quoting style to single quotes 
instead of the usual doubles.

Open this document in lyx-1.4  Notice how it is impossible to turn 
single-quoting off again, as lyx-1.4 don't have this in the GUI 
anymore.  The problem persists as the file is saved by lyx-1.4.

The correct solution:
Lyx 1.4 have no GUI for singlequote-mode, because single quotes are now 
inserted as special characters.  So support for singlequote-mode in the 
.lyx should be removed too (no saving/loading of this option, and no 
internal state variable to keep this state either), and lyx2lyx should 
simply strip this option when converting old documents.

This would solve the problem. Old documents with single quotes should 
still work, and users will be forced to insert single quotes using the 
insert menu.

Alternative solution:  Reinstate the button that switches between 
single/double quote mode.  Everything else is in place already - it was 
never removed. (Ideally, in single-quote mode the insert menu should 
then offer the insertion of a double quote as an alternative to the 
default single.)  This is the best solution _if_ singlequotes are 
popular anywhere in the world.

Helge Hafting