Spacebar / keyboard shortcuts break after using sub-dialogs or radiobuttons in the menubutton

[nekohayo]

1. Open a video such as the sample mentioned in No subtitles shown on this video sample #118
2. Notice that spacebar is able to play/pause
3. Click the menu button, navigate to the subtitles menu, and click a different subtitles track radiobutton

Alternatives to step 3:

• open the About dialog, then close it (note that any unimportant dialog should be dismissable with the Esc button, too…)
• open the file chooser dialog
• open the keyboard shortcuts dialog

Result: keyboard shortcuts don't work anymore, until you click the Play button on the toolbar.

I think I've seen this happen in other situations / other places in the app, but I don't remember which exactly. This is the easiest way to reproduce the issue.