Another history plugin based on sqlite.

Most important changes:

- based on sqlite3 instead of xml-files

- import history button in config dialog

- import option added for the other xml-based history-plugin

- pidgin import improved

- test and skip duplicates on import (so it is possible to synchronize
regularly from a windows-pidgin installation for example)

- more speed in most cases

The main reason I began to rewrite the complete plugin was, that I had
dataloss by importing from pidgin with the old history plugin.
The months, which where imported from pidgin, did overwrite the kopete
history. This was unacceptable.