Fix panel clock from not respecting 12/24 hour settings

plasma-settings changes the 12/24 hour settings by setting a config option in .config/kdeglobals. Switch the panel clock to checking the settings from there, and add a file watcher to update the format when the setting has been updated.

**In theory this should work, but I am not sure how to test this. If someone could test this, or give some instructions that'd be great :) **

Another note: I presume we also need to fix the lockscreen look-and-feel's clock as well, but I'm not sure if I can add cpp there...

Should fix https://invent.kde.org/plasma/plasma-phone-components/-/merge_requests/134