Improve code flexibility.
- Remove g_fnPrevProc, g_hDispWnd, g_hToolBar, and g_Anime global variables.
- Declare PREVIEW_DATA structure and use it.
- Encapsulate preview window by using user data.
CORE-19358
- Move Anime_GetFrameDelay and Anime_Step.
- Remove one parameter from ImageView_SaveSettings.
- Rename some preview-window functions as "Preview_...".
- Rename ImageView_CreateWindow as ImageView_Main.
- Use lParam of WM_CREATE.
CORE-19358