Create the notification system

[simonmicro]

Ohmmm, why did you delete all unit-tests except yours?

[akmal-threepointsix]

Ohmmm, why did you delete all unit-tests except yours?

Hello, I am the second contributor of this PR.

Were these unit tests necessary? I deleted them because:

• logging.cpp didn't compare obtained and expected results (no calls to EXPECT or ASSERT), so it didn't automatically test anything, just printed some text to console and litteted the actual tests outputs.
• string.cpp I deleted them because I was confused by String class which inherits from std:string (and it's not a good practice to inherit from std classes). But now, as I can see, we need this String class for Arduino, am I right? In this case we should return these tests, they seem good and useful.

[simonmicro]

* `logging.cpp` didn't compare obtained and expected results (no calls to `EXPECT` or `ASSERT`), so it didn't automatically test anything, just printed some text to console and litteted the actual tests outputs.

Yes, it just prints to the console, as I had never any motivation to capture the output and check it - so, yes it is not really testing... Except the fact that it test compile-ability (take a look at the chained templates inside the OswLogger-class), running without crashing and (if I look at them) "do they look right". Feel free to friend / inherit the OswLogger class and patch in the output redirect necessary, or open an issue - maybe I'll fix that... -> nvm, I did it myself #338

* `string.cpp` I deleted them because I was confused by `String` class which inherits from `std:string` (and it's not a good practice to inherit from std classes). But now, as I can see, we need this `String` class for Arduino, am I right? In this case we should return these tests, they seem good and useful.

Indeed - furthermore, the String class has a lot of custom (hacky) features to work like the Arduino ones and I broke it more than once by accident. Therefore there are tests - and while inheriting from std::string is not so cool, it allows the use of other libraries out there, which are not written for Arduino - take "ArduinoJson", which on the PC-platform only compiles with std::string support. Finally, I saved me a lot of work to re-implement another custom String-class by just reusing std::string :)

[simonmicro]

Regarding the missing unit-test checks for the logger: There you go (I've just had some spare time) -> https://github.com/Open-Smartwatch/open-smartwatch-os/pull/341