Select a better names for signal, var_signal, events and event_source

[YarikTH]

Description

signal is a bad name in C++ context, because 99 of 100 C++ programmers remember is as coordinated callback mechanism from Qt, boost etc, while ureact::signal is an observable value instead.

There are a lot of different name variants from different libraries: property, state, cell, value, behavior. It is hard to chose from them.

Apart from signal, events also suffers from bad naming. It is hard to refer both to singular events and for plural events. How to name several events? eventss eventses 🤮?

[YarikTH]

Original C++React author later renamed them into

• State
• StateVar
• Event
• EventSource

I like how State and Event have the same amount of chars in them and naming problems more or less solved.