Fallback to using the QSQLITE3 driver if QMYSQL can't be used

On SUSE Linux Enterprise Desktop (SLED) with the PackageHub repository
enabled (which provides the KDE packages) the mysql package is not available,
so there's no mysqld, and the QMYSQL driver can't be used.

OTOH, SUSE Linux Enterprise Server (SLES) with the same PackageHub repository
enabled has a mariadb package available which provides the mysqld binary.

Since the akonadi-server package for SLES and SLED is the same, we can't
just set DATABASE_BACKEND to SQLITE at buildtime and we need to choose
a fallback at runtime, which is what this commit implements.

If it's the first time akonadi starts and mysql is expected to be used, this
commit checks if it's really usable, and if it's not, it checks if the QSQLITE3 driver
is available and if it is, we fall back to it instead of using mysql.

Since we need to find out if a DbConfig* object can be initialized before
it's actually initialized, I added a bool storeSettings parameter to
DbConfig::init(QSettings &) that can be used to try the initialization
(find out available paths, etc.) without storing anything.

Then I added two methods DbConfigMysql::areRequirementsAvailable and
DbConfigSqlite::areRequirementsAvailable that check if the given object
has the system requirements available. I didn't add it to
DbConfigPostgresql because Postgresql isn't used by default in any case.

Also, note that this check is only done the first time akonadi starts
when it's not configured. Once it starts with mysql or sqlite3 this
configuration is stored and used the next times without any further
check.