Changes in SubtitleButton class (see description)

- Added two new factory methods: createPauseButton and createPlayButton
- Slightly tweaked createCloseButton and createMinimizeButton so:
- they now have ActionListeners if wanted by the using class
- Pixel values are now rounded down so the images look smoother
- Turned the part where 'a' is defined into a new method as all the
'create' methods share this part.