Remove mysqli use

This replaces mysqli use with PDO hardcoded to mysql.
Add OpenLayers 2.13.1
Use OpenLayers for map views

And the OSM nominatim service for geocoding, and the OSM default map.
Split overlay in person and group layer

Also fix up the OpenLayers design a bit
Make Google Maps use optional

The default is GMaps, but the example config sets up OSM.
In case no Google Maps are used, its javascript isn't loaded either.
Maps rework

- remove the person/group buttons.
  Instead, these are now separate layers

- make person layer a vector layer
  This allows collecting clusters of people
  into a single item with a counter

- hide some maps internals
Add addPersons interface to maps

This allows preparing a large set of persons to add in one go,
which performs much better on large data sets (eg. 10k entries)
Consolidate all db schema changes in a single file

Handle all schema migrations through a single switch statement
where updates fall through. No need for loops or magic function
invocations.

The schema for the initial database setup was reverse engineered
by taking the old schema_install.sql (which was equivalent to v2.36's
schema) and unapplying all older changes as relevant (column type
changes were ignored since they become no-ops)

While at it, fix support for table name prefix: Both the database
initialization and later update steps failed here.