Move shared frontend code from SDL to a seperate project

src/frontend-common/imgui_styles.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <imgui.h>
+
+namespace ImGui {
+void StyleColorsDarker(ImGuiStyle* dst = nullptr);
+void AddRobotoRegularFont(float size = 15.0f);
+}