Enhancements for --export-type command line option

Now allows to

• export multiple types at once, e.g. using --export-type=pdf,png
• export to different file name(s) when using --export-type by using --export-file option (extension is stripped/added as needed)

Also be more lenient with user-specified file extensions in general: Warn the user, however do not abort export but continue with what we would have expected (and the user likely meant to do)

Some examples:

# export a single input file to two export formats in one command
inkscape --export-type=png,pdf in.svg

# export a single input file to two export formats and rename them in the process (will produce out.png and out.pdf)
inkscape --export-type=png,pdf --export-file=out in.svg

# export two files to two export formats
inkscape --export-type=png,pdf in1.svg in2.svg

# do not error out due to wrong output file's extension but warn the user and write to out.png instead
inkscape --export-type=png --export-file=out.pdf in.svg