Added subtitles-video linking/unlinking.

Now subtitles can be linked with video, i.e. subtitle videoPath can be changed. FPS from linked
video can be easily detected. Changing linked video doesn't cause automatich FPS detection from the
new one though.

This commit resolves issue (feature request) #61.
GUI readability improvements

- Added some tooltips.
- Moved adding/removing subtitles higher in the menu.
Store latest visited dir in state.ini

Latest dir is not a program setting (from user point of view), but an information about latest
program state. Therefore it should be stored in state.ini (which users shouldn't modify), not
subconvert.ini.