DRY in FSStoragePlugin::handleFSCreate()
Fix -Wunused-but-set-variable in FsStoragePlugin
Don't prune playlist files on filesystem

FsStoragePlugin used to scan the whole exported storage for playlists
all of them unknown to tracker were deleted. This isn't very desired,
particularly in conjunction with removable SD cards where we could end
up unwittingly deleting user's own data.
Factor out part of repeating code into FSStoragePlugin::addItemToMaps()
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