gui: Add option to set timezone minutes offset & Fix wrong timezone hour offset & Add timezone dst support

[bkerler]

This fixes the wrong timezone hour offset range (did include only +12 but not up to +14 hours) and also introduces a timezone minutes option to select a timezone offset in minutes (0 min, 30 min and 45 min).

This addresses the issues #3550, #1779 and #3141.

As the timezone eeprom storage value is only uint8_t, the data could be further optimized and compressed into a bitstream as it also saves space, which would be possible.

My recommendation for storing the timezone data compressed in a uint8_t:
struct tz_offset{
uint8_t hours:4; // 0..14 hours offset
uint8_t mins:2; // 0=0, 1=30, 2=45 minutes offset
uint8_t summertime:1; // 0=wintertime, 1=summertime
uint8_t signed:1; // 0=negative gmt hour offset, 1=positive gmt hour offset
};

If that is wanted/needed, I can also add these needed eeprom changes as well to use a compressed structure instead.

[jurriaan]

Not sure how space-limited this firmware is, but embedding something like https://github.com/evq/utz would be even nicer so we would be able select the time zone and have all offsets + DST applied automatically.

[bkerler]

Not sure how space-limited this firmware is, but embedding something like https://github.com/evq/utz would be even nicer so we would be able select the time zone and have all offsets + DST applied automatically.

I tried to also implement a list-based approach using a list of countries / timezone settings (without libraries). The problem is, that this would be static (353 and more timezones that change quite often) and very firmware space consuming. Especially for the mini printers it would cause huge issues because of missing space.