Cleaned up some string literals

Learned string literals are concatenated automatically so I used that to clean up some strings.
bugfix: empty spinboxes

Trying apgtk out on debian unstable showed that the spinboxes would not be populated without an appropriate signal.

Unknown if this is a bug in my program or a bug in gtk/gi in deb unstable but there's no harm in fixing it anyway.
Added generic error handling to generate event