Refactor StorageItem creation in FSStoragePlugin

Method addToStorage() is now the single point where cration and
initialization of StorageItem objects happens. This allowed to remove
some repeating code and create a unified way new items are added
to the storage.
Fix FSStoragePlugin_test::testInotifyCreate()

The test case inadvertently modified m_pathNamesMap. After refactoring
in FSStoragePlugin this started to matter.
Pass destination storage instance into StoragePlugin::moveObject()
Ensure correct storageId in FSStoragePlugin::addToStorage()

If we get MTPObjectInfo as an argument.
FSStoragePlugin::addToStorage(): allow assigning custom handle
Fix utterly misleading StoragePlugin::getObjectHandles() documentation
Remove documentation of inherited members from FsStoragePlugin

Doxygen automatically inherits the text from parent class when
generating documentation (INHERIT_DOCS has to be set YES in Doxyfile),
so maintaining it in two places is just a needless hassle.
Factor out common method StoragePlugin::copyData()
Fix FSStoragePlugin::moveObject() for cross-storage moving
Add test cases for inter-storage object movement