WIP: Add shapes through buttons

[albfan]

Build and run from flatpak with:

flatpak-builder --force-clean flatpak-build com.github.akira.libgtkcanvas.json        
flatpak-builder --run flatpak-build com.github.akira.libgtkcanvas.json gtkcanvas-demo 

TODO:

• Read attributes from Document (should be fixed building svg from file)
• Allow to select objects behind other in its transparent area (supposed to work from gsvgtk master)

[albfan]

Although this is not the correct approach anymore, I fixed the version dependencies on flatpak so we can always revisit for importing svg and edit properties