- Buttons events source as button property instead of a global one
- Rename events source -> provider for all settings, consistent with the other things like relay and light providers
- AnalogPin to read between a certain analogRead() range
Trying to follow defaults here - analog 'press' is digital LOW, default value is HIGH, so no additional cfg entries are needed besides pin, level and changing evt source
- (debug) Refactor gpio command, add adc to show analogRead(pin)
- (debug) Add button command

Implemented based on:
https://gitter.im/tinkerman-cat/espurna?at=5f5d44c8df4af236f902e25d
https://gitter.im/tinkerman-cat/espurna?at=5f60e7f1f969413294e95370

- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags 
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE

* wip

* cleanup buttons-through-serial code, remove hw mention from button module

* remove wip

* implement mqtt settings

* fixup struct members, dual no longer allocates debouncer

* add missing debounceevent lib

* fix missing event_handler, update names

* fix namespace

* drop lib

* fix int<->bool comparison

* Move gpio16 handling from DigitalPin to EventHandler

* Cleanup debounceevent headers

* Don't expect system headers to be included

* re 70b54c489f - no allocation, for real

* Adjust settings names

* dont retain by default

* unused

* typo

* Fix length type (ref 6017ad9474)

* Move pin class outside of debounce lib, lowercase ns

* move event handling inside of button_t

* refactor config. ..._MODE -> _CONFIG, ..._MODE_... -> ..._ACTION_...

* fix test

* naming

* move indexed value to header

* refactor actions into direct opts

* fix webui, fix buttons not respecting old user setting

* change button config format from bitmask to a struct, adjust settings conversion

* proxy some more header defautls, fix web kv

* gpiopin

* adjust webui func to support every setting

* clarify single-return event->string

* fix dual setting

* fix dual packet condition, de-duplicate funcs

* fix bogus warning

With uint8_t length function _buttonMapReleased never returned BUTTON_EVENT_LNGCLICK. BUTTON_EVENT_CLICK returned instesd. I verified that with uint16_t length it returns BUTTON_EVENT_LNGCLICK when correspondent delay passed.

- use similar to relay & wifi, configuration through helper methods that
  use indexed defines
- clock cycles for led delay polling
- button mode defaults