Namespace build configurations of modules, make more things into constexpr
(not fully finished though)

Unify code using ...Count() to parse IDs

Avoid using unsigned char aka uint8_t as index, prefer size_t
as most code already uses it anyway. Making sure we never accidentally
truncate the value or try to read it as 32bit-wide. Also, simplify
access to built in containers, since those use the wide type as well.

Renames led and button types, more consistent initialization and field access.

- 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

* support GPIO setting in WebUI
* experimental led pattern support
* support loading settings in setup() (not yet fully working)