filter calendar objects by component type (VEVENT, VTODO or VJOURNAL)
add url to calendar object as information
unit test for component filter in ICSExportPlugin
calendar object url is accessible in mergeObjects
prevent a double-slash / remove unneeded slash
Revert "prevent a double-slash / remove unneeded slash"

This reverts commit 532a76d38c1044b5bfda764fde14f4ad88771fba.
Merge pull request #568 from armin-hackmann/icsexport-component-filter

ICS Export: export by componentType