Cmake build: Try to omit using directory names as source-files in po

Cmake build: Try to omit using directory names as source-files in po

Previously there are po-entries like
  #: lib/examples/Articles:0 src/TocBackend.cpp:296
  #:src/frontends/qt/GuiExternal.cpp:87
  msgid "External Material"
  msgstr ""

now:
  #: src/TocBackend.cpp:296 src/frontends/qt/GuiExternal.cpp:87
  #: lib/examples/Articles:0
  msgid "External Material"
  msgstr ""

(The string at 'lib/examples/Articles:0' proceeds also from the directory name 'lib/examples/External_Material')

This makes it easier to use some po-editors like 'linguist'