gui: add model load and save positions

This patch enables to load and save irt file with position (a couple of float)
for each model (closes: #42).
1 job for master in 1 minute and 7 seconds
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[15/24] Building CXX object app/gui/CMakeFiles/irritator-gui.dir/__/__/external/imgui/imgui_widgets.cpp.o
[16/24] Building CXX object lib/CMakeFiles/simulations.dir/test/simulations.cpp.o
[17/24] Building CXX object app/gui/CMakeFiles/irritator-gui.dir/simulation-editor.cpp.o
[18/24] Building CXX object app/gui/CMakeFiles/irritator-gui.dir/sources.cpp.o
[19/24] Building CXX object lib/CMakeFiles/test-api.dir/test/public-api.cpp.o
[20/24] Building CXX object app/gui/CMakeFiles/irritator-gui.dir/node-editor.cpp.o
ninja: build stopped: subcommand failed.
Cleaning up file based variables
ERROR: Job failed: exit code 1