imgui: migrate to new event-based IO (#604)

## Link to GitHub Issue or related Pull Request, if one exists
#287

## Description of change
Switch over to new event-based I/O system in ImGui so that we can keep
up with ImGui updates.

Unlike #287, this change does not modify how mouse position is handled.
That continues to be supported by newer versions of ImGui.

## Testing
wip

src/spice2x/overlay/windows/iopanel_ddr.cpp

@@ -40,13 +40,17 @@ namespace overlay::windows {
         ImGui::Dummy(ImVec2(12, 0));
 
         ImGui::SameLine();
+        ImGui::PushID("P1");
         this->draw_buttons(0);
+        ImGui::PopID();
 
         ImGui::SameLine();
         ImGui::Dummy(ImVec2(12, 0));
 
         ImGui::SameLine();
+        ImGui::PushID("P2");
         this->draw_buttons(1);
+        ImGui::PopID();
     }
 
     void DDRIOPanel::draw_buttons(const int p) {