- provide a generic way to read default status of the pin after boot
  allows us to use switch as both on and off, independent of the position on boot
- add BUTTON_DEFAULT_LOW & BUTTON_DEFAULT_BOOT (now it's not just BUTTON_DEFAULT_HIGH)
  wiki described raw numbers for some reason :/ this will break that
- clean-up 'constexpr const' and 'const' function args, both are redundant
- clean-up debounce event member defaults, put things in the header

- Add ::Released button event to be used specifically with switches, opposite of ::Pressed
- Always use default state for switches instead of ignoring it and using current pin reading as value. This allows us to boot with the switch held in the opposite state and trigger the correct event immediately
(meaning, switches now depend on the BUTTON_DEFAULT_HIGH presence to detect the default position)
- BREAKING: Fixup hardware.h. Any custom board needs manual changes.
- BREAKING: Rework MQTT button messages to send text from debug messages instead of numeric code

- 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