Quickfixes: Review quickfixes for 0.5.0 compiler warnings/errors

[flantony]

Derived from : #132

In general 0.5.0 will be stricter. This affects function visibility, new and disabled keywords and stricter type checking. Therefore we might face new compiler warnings and we need to determine in which cases we can provide helpful quickfixes.

TODO:

• Identify new compiler warnings and analyse if we can provide a quickfix.
• add new Warnings to com.yakindu.solidity.compiler.builder.processor.SolidityWarning
• determine whether it is necessary to introduce solidity-version specific quickfixes
• Create new issues for new Quickfixes