Improve some of the breve/caron/cedilla mappings
* lisp/leim/quail/latin-pre.el ("latin-prefix"): Describe new 'c with cedilla mapping'. Extend caron description. Correct dot above description. Add new mappings for 'a with breve' and 'c with caron'. Add additional mapping for 'c with cedilla'. Delete ~o mapping for 'g with dot above'.