Fix clean build

fsstorageplugin.so now depends on libmeegomtp.so, so
mts_fsstorage_plugin target has to depend on mtpserver.
Add inotify watch to all directories

By mistake, addWatchDescriptor() was called only when the directory was
being created in FSStoragePlugin::addToStorage().

Reworked FSStoragePlugin_test::testInotifyCreate() to cover this bug.