[FS#73] Add text alignment choice in GUI.
Proof of concept of sheet export as TikZ.
Dpi and dimensions in pixels are stored directly in the sheet.

This change was needed to enable usage of .dpix2coo() method
when exporting as tikz.
Remove constants DEBUT, MILIEU and FIN. Fix major regression from last commit.
[FS#226] Replace unpythonic int constants (like NOM) by strings ('nom' here).
Fix many issues with commit d452ad4c (ratio and window management in sheet).
Fix angles labels display.
Intersections can't be selected since they can't be moved.
[geolib] Centre and Centre_gravite are now synonyms.
First working implementation of tikz backend.