gui: add menu to clear and add predefined models

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      |         ~~~^~~~
../lib/include/irritator/core.hpp:815:20: note: declared private here
815 | node_type* node{ nullptr };
| ^~~~
[11/18] Building CXX object app/gui/CMakeFiles/irritator-gui.dir/__/__/external/imgui/imgui_widgets.cpp.o
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ERROR: Job failed: exit code 1