[msyncd] Fix various issues in OOPP support

Some enum types needed to be registered properly with Qt's metatype
system in order to be used via DBus.

Fixed the signatures of various signals and slots.

Increased the DBus timeout amount to allow for slow plugins.

Properly wait for results from client plugin processes to become
available before checking the validity of those results.

Ensure that client plugins are initialized before calling their
cleanUp() function to avoid crash.