CMakeL10n: Avoid option --use-first for calls msguniq and msgcat #76

Using the --use-first option causes that comments and location information is used only from the first occurrence and all others are discarded instead of merging this information together.

When the information are merged, the gettext inserts an ugly comment that referring to the POT file from which the information comes from. These in our case are commonly temporary files and therefore such information is not only ugly, but at the same time unnecessary. That's why I added the removal of this ugly comment.

It seems that it is good for use with msguniq with POT for desktop files, but it is not possible to use with msgcat when connecting other POT files. I have to do more tests.

The problem for joining the POT files represents POT header. I adjusted the code so that instead of using msgcat call, files are joined in CMake code and then are removed all repeats of the POT header. As a result, I did simplify the code so that a single msguniq call is now used.

Note: Because call of msguniq is now always performed as part of the POT file finalization and at the time when the working extension of the file names is cleaned in references to source files, this can lead to changes of the line breaks in POT files. If you do tests of this PR, you can observe such changes in POT files.