Load the adblock settings in background.

If you open rekonq and start typing immediately, there is a small lag
(about half a second).
This is caused by the AdBlockManager::loadSettings function reading
the adblock rules. It is called with a one second delay to make the
startup faster. With this patch, the function is called in a separate
thread using QtConcurrent::run to eliminate the lag.

To make this safe, the adblocker is enabled after the settings are
loaded, which means, that the first site loaded could be partially
with ads. But this is no change in behavior, as the adblocker in
current master is disabled for a whole second.

The only times this blocks now is in situations, where it is necessary
that the settings are loaded (showSettings and addCustomRule), but I
guess that no normal user will use those in under
a second after startup :)

REVIEW: 111712

REVIEWED-BY: adjam