Efx improvements

- speed of the preview area reflects speed settings
- all fixtures are displayed in the preview area
- direction of the fixture movement is based on both EFX direction and fixture Reverse setting (so far only the latter was counted)
- added tests for backward movement
- added "Start Offset" parameter (0-360) to both EFX (global) and fixtures (individual). Allows starting the movement in any point of the path

Note that I have not tested it on an actual moving head(s) yet.