[Feature] 24-hour clock

[Gaga889]

Platform: Linux (flatpak)

Adding a setting to toggle between 12-hour and 24-hour clock for the message timestamps.

Alternative: use the system locale setting (LC_TIME) to automaticaly determine time format as fallback (not sure if that is possible with flatpak sandbox.

[Gaga889]

After a bit of testing:

Current implementation:

short timestamps:
ALWAYS 12-hour clock

long timestamps (popping up after hovering over the time):
follows the locale (LC_MESSAGES)

i think it would be more intuitive with:

short timestamps:
toggle in settings and locale (LC_TIME) as fallback

long timestamps:
follows the locale (LC_TIME) and (LC_MESSAGES) as fallback

as stated above: not sure if this is a good implementation for a flatpak