Resolve "Raise warning level and fix warnings on MSVC"

This MR resolves all warnings at level 3 and quite a few of the warnings at level 4 of MSVC.

The variable renames are mostly because the old names shadows a (member) variable. (possibly of a base class; like the member p of QList). This isn't resolved throughout the whole code base, since this shadowing warning is part of W4.

Closes #188 (closed)