1. Prepare Pure and Bell specific `Application`
classes and add them to `app` target:
- `products/BellHybrid/apps/Application.cpp`
- `products/PurePhone/apps/Application.cpp`
2. Update `CMakeLists.txt` files.
3. Move `ApplicationBell` implementation to Bell-specific
`Application` class and remove `ApplicationBell` files.
4. Change Bell apps parent classes from `ApplicationBell`
to Bell-specific `Application` class.
5. Rename `Application` to `ApplicationCommon` in the rest of the files.
Modify calculator behavior according to PO input:
* Compute existing equation before entering next operation
* Limit digit input to 7 digits
* Start each number input from empty input
1. Fix possibility to write illogical operations.
2. Fix possibility to write multiple '0' when it is not needed.
3. Add scientific notation when result do not fit on screen.
Reduce firmware size by refactoring global data defined in public
headers.
Each global variable which require runtime initialization adds
initialization code to every translation unit which includes the header
where the variable is defined and declared.
