Fields: Code Cleanup: move non-field functionality to other file

[henrygab]

Originally posted by @tobi01001 in https://github.com/jasoncoon/esp8266-fastled-webserver/pull/216#discussion_r757542649

In particular, the following should be moved elsewhere:

• [] power variable
• [] brightness variable
• [] setShowClock() function
• [] setClockBackgroundFade() function
• [] setUtcOffsetIndex() function

Consider also making some function templates to further consolidate the functions for get/set of the variables. This should reduce bugs when needing to add new functionality (such as websocket transmissions on value changes), for example.