Change default shortcut for previous/next

This changes the shortcuts for Previous/Next line from Shift+Left to Ctrl+Shift+Up and from Shift+Right to Ctrl+Shift+Down. This way, one can easily jump between lines even when the focus is currently in the text editor.

This does not conflict with standard selection tools (selecting words) and it makes sense given the subtitle lines and the waveform are both up/down oriented.