catch SIGSEGV in order to prevent user from expecting capture when kazam crashed right after capture start

[Karl-Philipp Richter]

If the user start the capture with the capture button and kazam crashed due to SIGSEGV, it's quite hard to find out that there's no capture at all. If the user does a video tutorial for 40 minutes it's quite annoying to find out that kazam didn't do anything. The only way to tell if kazam has crashed is through observation of the tray icon (output of a terminal is usually only inspected after the end of capture). Catching SIGSEGV and displaying a warning dialog if it is sent, would help a lot.