Improve image lib and tool

Improvement:

- Add "fit" option with symbol `f` which will automatically detect the new image size and resize the image proportionally to remove white space.
- Apply image resize option above to model/tool/image in format: `$this->model_tool_image->resize($image, $new_width, $new_height, $resize_type)`
- Fix watermark different image type issue (image .jpg while watermark .png) by allowing to set mime on lib/image->create()
- Add new watermark position "center" and apply as default watermark position in switch statement
- Add watermark offset option to increase visibility watermark image position not in the edge of main image
- Add new method `imagecopymerge_alpha()` to fix .png transparancy issue (used by watermark).

Note: watermark is not yet applied to `$this->model_tool_image->resize`