Fixes #1656

Following Jeremie's idea #1656 (comment), I've introduced a new "parent" ImGuiPlugin which handles ImGui IO functions and converted the old ImGuiMenu, ImGuizmoPlugin, SelectionPlugin plugins to inherit from a new ImGuiWidget class. In this way, a single ImGuiPlugin can be registered to the viewer's plugins list and one or many widgets are registered to this plugin's widgets list.

For folks just using the menu, this incurs a few extra lines. To add the menu:

#include <igl/opengl/glfw/imgui/ImGuiPlugin.h>
#include <igl/opengl/glfw/imgui/ImGuiMenu.h>
…
igl::opengl::glfw::imgui::ImGuiPlugin plugin;
viewer.plugins.push_back(&plugin);
igl::opengl::glfw::imgui::ImGuiMenu menu;
plugin.widgets.push_back(&menu);

The reward is that now we can use all of these plugins (and any new user created plugins using ImGui subroutines) without conflict.

To highlight this, the updated 109_ImGuizmo successfully shows the Viewer menu and the ImGuizmo Widget

(since this is a breaking change. I'd love to get it into the upcoming 2.4.0 release)