Backported Time-Sync into upstream

[dkosmari]

I didn't touch the README.md nor the version in Makefile. Remember to modify them later, as well as any item name (in cfg.cpp) and item ordering (in config_screen.cpp) changes.

UI changes:

• Pressing X on a config item resets it back to the actual default value, not to the previous value.
• Configuration items now support holding the button down for repeated events.
• Timezone offset is now set trough a single item, in a more natural format.
• The preview clock is now updated in real time while it's focused, thanks to the new WUPS API.
• Some tweaks to config items, like better text scrolling, and boolean items can always be toggled with left/right.

New features:

• Timezone offset/DST can be updated automatically (through a HTTP query to ip-api) when the plugin runs.
• Number of background threads can be configured. This controls how many NTP servers are queried at the same time.
• More control over notification verbosity: it's now possible to have a non-verbose level of notifications, that only reports when the clock is adjusted, or if total failure (no NTP server could be used) happened. Users that need troubleshooting NTP/network problems can enable the "verbose" mode to see more details.
• Opening the config menu reloads the settings from the JSON file. This makes it more convenient to edit the file through FTP, when the user wants to use a custom NTP server.

[Nightkingale]

As always, thank you for your contributions! It is greatly appreciated, and helps make this plugin the best it can be!