Upgrade wallet to support QT 6

[olemis]

We need to:

• [ ] Drop therecipe to use Qt official releases for all Qt-related code
  • [x] UI files do not have any dependences with therecipe
  • [ ] A new Go API (libfibercrypto) exported as a C++ wrapper will be used to call Go functions from C++
  • [ ] Qt models are implemented in C++
    • [ ] Qt models do not have any dependence with therecipe
    • [ ] Qt models do not have any dependence with Go-specific code
      • [ ] Refactor Qt models to make them independent of Go-specific code
  • [ ] Utilities that use Qt libs are implemented in C++
  • [ ] The program's entry point (main) is implemented in C++

ETA => 12/2020

[LateusBetelgeuse]

https://github.com/fibercrypto/fibercryptowallet/issues/363 is a blocker for this issue. All features to help developers to transition from Qt 5 to Qt 6 are in Qt 5.14 and Qt 5.15, and no new features were added to the wrapper since Qt 5.13.