Use platform windows for windowing instead of custom draggable window system

src/ui/ui_popups.h

@@ -0,0 +1,31 @@
+#pragma once
+#include "ui/ui_core.h"
+#include "ui/ui_widgets.h"
+#include "platform/platform.h"
+#include "renderer/renderer.h"
+
+#define MAX_POPUP_WINDOWS 4
+
+struct PopupWindow {
+    B32              alive;
+    B32             *open_flag;        // e.g. &app->show_settings_window
+    PlatformWindow  *platform_window;
+    Renderer        *renderer;
+    UI_Context      *ui_ctx;
+    UI_WidgetState   wstate;
+    UI_WindowContentFn content_fn;
+    void            *content_user_data;
+    S32              width, height;
+    S32              last_w, last_h;
+    const char      *title;
+};
+
+PopupWindow *popup_open(PlatformWindow *parent_window, Renderer *parent_renderer,
+                        const char *title, B32 *open_flag,
+                        S32 width, S32 height,
+                        UI_WindowContentFn content_fn, void *user_data);
+void         popup_close(PopupWindow *popup);
+PopupWindow *popup_find_by_flag(B32 *flag);
+void         popup_do_frame(PopupWindow *popup, F32 dt);
+void         popup_close_all(void);
+void         popup_close_check(void);