Refactor and improve the PGF/TikZ exporter of Kig

Changes:
- Put the TikZ exporter class into its own file (to clean up the long
latexexporter.cc file)
- Checked and rewroked all the visit(*) methods and the TikZ code
generated
- Implemented "point" object sizes
- Fixed arros heads which were wrongly added to all lines/curves
- Fixed too long lines in the output of generic curves
- Probably some more minor things

Note: This is work in progress
- TODO: More finetuning of: linewidth, line styles, point styles

On behalf of Raoul Bourquin


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