Fixed exceptions thrown on changing of  playing movie.
Added drag and dropping files on Subconvert window to open them.

Both movies and subtitles can be opened this way.
This implements request #59
Fixed loading dragged and dropped movies.
Added FPS info to file list.
Improved version control

subconvert_version.sh script checks for Subconvert version in the following order:
1) .tarball-version file contents (only generated for dist tarballs)
2) git describe
3) fallback version (currently 1.x-unknown)

Additionaly, it is invoked every time make is called and version.py __version__ variable is changed
with sed (unfortunate hack). It means that version.py will not take into account @PACKAGE_VERSION@
variable.
Removed generated "*.py" files from gitignore.
Added videoPath to subtitle data

VideoPath reflects a video linked to loaded subtitle (not really relevant in CLI mode). User will be
able to easily fetch FPS from such video + it might be saved in some subtitle formats metadata.
Getting FPS from linked files might be placed e.g. in file list context menu.

Linking of video and subtitle might be triggered by the following conditions:
- when loading subtitles. Subconvert automatically tries to match subtitle filename base with known
  video formats. Currently videoPath is saved "forever". If video is found and FPS is successfully
  loaded, an icon will appear near FPS value on file list.
- user input (to be implemented). User might link or unlink files with any video file (chosen from
  FileDialog).
Disable video slider when video is not loaded

This commit also fixes exceptions that are thrown when there is no FPS available when changing
slider positions. This may occur e.g. when no video (or incorrect one) is loaded or when FPS data
wasn't found in loaded video.