Adding core340.xml to deal with coming release. Letting customise this reference in configuration

The core(xxx).xml is used 
1. To add the correct core icon in the Translations manager
2. when creating extensions packages in order to not propose core files to the selection.
To test, install the French pack which contains geshi.
Then edit media/com_localise/packages/core340.xml and add 
`<filename>plg_geshi</filename>' in the administrator group.
before patch: the file will be available when creating an extension package
after patch, it will not show anymore in the list