Use td.orm_session() around all other database accesses
WIP: new stock item picker

New stock item picker is used for annotations and record waste

It still needs shortcuts adding for line keys
Use new stockpicker for "finish stock not currently on sale"
Remove "multiple keyboard layouts" feature

Multiple keyboard layouts are now supported by using different ranges of
line numbers in the till configuration file.  There's no need to treat
them differently in the main body of the code.