implement smart-append for ki18n_install cmake editing

this feature was originally not implemented for the new function introduced
to support the new ki18n_install macro.
ki18n_install(subdir) is now only appended iff there is no such call for
the same subdir already.
additionally #FOO_SUBDIR (e.g. #PO_SUBDIR) can be used to explicitly mark
where the macro call should be placed if releaseme is supposed to handle it

all cases are test covered now as well