Remove Input combibox

[chenlung]

• The user must choose an FPS upon the import of formats in which its timebase is unclear (e.g. millisecond-based files such as SRT and WebVTT).
• When creating a subtitle file, I propose that the user be prompted to select one.

Because the Input combibox supposedly has no real practical effect once a subtitle set is loaded or created, I propose that it be removed. Instead, we just list the FPS of the project elsewhere for informational purposes (see this).

Also consider this.