The internal management of layers was based on localized layer names.
So it could happen, that a custom layer name was a standard layer
name (which includes special handling) in one local and not in the
other. The patch changes the implementation to use the same real
layer names as in file, and use localized standard layer names only
for display in the UI.
Removes no longer needed RestoreLayerName()
Removes convertToInternalName(), ersatz convertToLocalizedName()
Removes no longer needed convertToExternalName()
Includes fix for tdf#119956 in addition

Change-Id: Iaedb73e4017e728f7ca3b0dc4aa7bb7f0e6ce6da
Reviewed-on: https://gerrit.libreoffice.org/60507
Tested-by: Jenkins
Reviewed-by: Regina Henschel <rb.henschel@t-online.de>