Add -Wsign-compare and -Wconversion flags

[lplewa]

On windows some of those problems are detected - what causes fully working on Linux PR to fail on Windows, what is annoying. Unfortunately, those Flags are more restrictive then on windows (by adding those flags we have to fix ~40 warnings in the code)

Question: Do we want to include those compiler flags?

[bratpiorka]

I am for adding as many checks as possible :) at least if this costs us little effort. Fixing 40 easy warnings should be straightforward.