Initial PlatformIO project for TTGO-T-Watch-2020 based of SimpleWatch example

A BLE UART device is advertised to emulate a bangle.js/Espruino to receive json messages from Gadgetbridge. This allows receiving all notifications from an Android phone.

Time updates from Gadgetbridge are parsed and applied to the RTC and system time.

Gadgetbridge notify messages are shown on screen in a popup message. Other message types are not yet handled.

gui.cpp was refactored slightly to separate GUI header and class implementation. Class definitions are now in gui.h so that other files may reference the GUI classes.