[nemo-qml-plugins-social] By default node caching is disabled. It's poss...

I'm not very proud of this, but I'm not really sure how to fix crash caused by node caching. For a workaround, caching is disabled by default and there is a new property "cacheMode" which can be used for enabling and disabling caching. By default it disables it.

Please review the previous PR about Undefined reference crash fix also and apply that too (if you'll accept it).