Improved options for calling tde_create_translated_desktop #57

Add the ability to call for the same source file installed in another location.
Process the source file using configure_file if it has a .cmake extension.
PO_DIR can be specified as relative.
Add the ability to process multiple source files to be installed in the same destination folder.

Thanks to these changes, the call originally looked as follows:

foreach( _desktop
    linkProgram.desktop linkURL.desktop linkFloppy.desktop
    linkHD.desktop linkCDROM.desktop Directory.desktop
    TextFile.desktop HTMLFile.desktop linkZIP.desktop
    linkDVDROM.desktop linkCAMERA.desktop linkNFS.desktop
    linkCDWRITER.desktop linkMO.desktop
)
  tde_create_translated_desktop(
    SOURCE ${_desktop}
    DESTINATION ${TEMPLATES_INSTALL_DIR}
    PO_DIR ${CMAKE_SOURCE_DIR}/translations/desktop_files/kdesktop-icons
  )
endforeach()

It can now look like this:

tde_create_translated_desktop(
  SOURCE
    linkProgram.desktop linkURL.desktop linkFloppy.desktop
    linkHD.desktop linkCDROM.desktop Directory.desktop
    TextFile.desktop HTMLFile.desktop linkZIP.desktop
    linkDVDROM.desktop linkCAMERA.desktop linkNFS.desktop
    linkCDWRITER.desktop linkMO.desktop
  DESTINATION ${TEMPLATES_INSTALL_DIR}
  PO_DIR kdesktop-icons
)

Modifications are made so that they do not affect compatibility for existing calls. Anyway, before the final merge there I'll have to do the merge with improvements to the use of LINGUAS made in #55.

Excellent work!