Add PLUGINDIR cmake setting and define #7027

Add PLUGINDIR cmake setting and define #7027

By default, plugins are only searched next to the binary or next to the
other Qt plugins. This optional build variable allows another path to be
configured.

The idea is that on linux the oC packaging probably wants the binary in
something like /opt/owncloud/bin and the plugins in
/opt/owncloud/lib/plugins.

Similarly, distribution packagers probably don't want the plugins next
to the binary or next to the other Qt plugins. This flag allows them to
configure another path that the executable will look in.