[qtcontacts-sqlite] Provide hook to debug SQL expansion of filters

Define QTCONTACTS_SQLITE_DEBUG_FILTERS to a non-empty value to report
the SQL expansion of contact filters.
Merge pull request #25 from matthewvogt/debug

[qtcontacts-sqlite] Provide hook to debug SQL expansion of filters
[qtcontacts-sqlite] Add missing include
[qtcontacts-sqlite] Add conversion for other subType fields

Using qtpim, convert between text and integer forms on insertion
and extraction.
[qtcontacts-sqlite] Add conversion for OnlineAccount's protocol field

Using qtpim, convert between text and integer forms on insertion
and extraction.
[qtcontacts-sqlite] Create shared implementation of conversion functions
[qtcontacts-sqlite] Convert filter data when required

Data used as filter values may need conversion to string form to be
sensibly compared.