PostProcessor and PostProcessorAllOutput (not finished)

The post processors work on a GInputStream obtained by the GSubprocess.
The GInputStream is read asynchronously.

Before, the Vala code worked on the full command output, as one big
string. This big string was split into lines, and then processed.

One exception: the latexmk post-processor worked on the full output with
a big regex, without splitting the output into lines (splitting the
lines was delegated to the all-output and latex post-processors, called
by the latexmk post-processor after extracting the relevant contents
from the big string).

Next steps:
- write the latex post-processor (the Vala code works already line by
line, so working on the stream will use the same algorithm). One
difference though: the latex post-processor should read the log file,
not the command output.

- write the latexmk post-processor. Working on the stream (line by line)
will be more complicated, the big regex cannot be reused (unless the
command output is constructed to obtain one big string, but I don't want
this solution, it uses more memory and is thus most probably slower).

- write unit tests!