General Code clean-up

Summary:
simplify if condition

always initialize variable

directly initialize variables

remove dead code

reduce variable scope, remove dead comments

remove declared, but unimplemented function remove unused global variable

removed unused member, removed member which was only ever read, never written

remove unnecessary forward declaration

remove declaration of struct that is defined below

remove redundant cast convert malloc + memset to calloc

use bool literals (clang-tidy)

use nullptr instead of 0 (clang-tidy)

Test Plan:
Manual regression testing: code still compiles, games still run, and solver can
win the game

Reviewers: KDE Games, aacid

Reviewed By: aacid

Subscribers: schwarzer, KDE Games

Differential Revision: https://phabricator.kde.org/D11779