Added @property decorator to some methods and renamed SubtitleManager test suite file.

Affected methods:
Subtitle.start
Subtitle.end
Subtitle.text
SubParser.getResults() -> SubParser.results
SubParser.isParsed
Subtitle.isEmpty -> Subtitle.empty()
FrameTime.getFps() -> FrameTime.fps
FrameTime.changeFps() -> FrameTime.fps = x
FrameTime.frame
FrameTime.fullSeconds
FrameTime.time
Added Subtitle tests + fixes.
Subtitle now handles FPS changing.

FPS changing includes synchronization of FPS for start time and end time. If anytime those two
differ, an exception shall be raised. Subtitle can also report its because end FPS and start FPS
are synced.