More features

I've finished implementing the support for changing the Frame Rates and subtitle's Style.

Frame Rates include the subtitle file's frame rate (Input FrameRate) and the movie's frame rate. Both can be set when using frame based subtitles, but only Movie Frame Rate can be set when using time based subtitles.

Subtitle Style includes Bold, Italic and Underline.

The multilevel Undo/Redo also includes these new features.