Increase code quality by getting rid of compiler warnings

[DerTomm]

The purpose of this PR is to increase the overall code quality by fixing the causes of current compiler warnings.

@A-A-G I noticed you also made these changes in your PlatformIO branch - but without a PR yet. This PR will make them generally available in trunk for further development.

Notes:

• It is possible that some of the changes are already part of other open PRs. However these PRs are either stale or address the wrong target branch (master).

• The contained commits f151599 and 115acbf are already part of PR #53 and just contained again for sake of compatible dev environment while that PR is still pending.

[AlexanderWillner]

Thanks! Integrated them into https://github.com/AlexanderWillner/TonUINO/commit/66f74d23c3e8542b6c31a8c0f6113e53ea4dd89a