feat: Allow installing into a prefix

Carl Schwanrequested to merge
ognarb/abstract-games:prefix-install into master

This allows using a prefix to choose where to install the games. It use extra-cmake-modules.

Example usage

mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=~/.local/kde -DPLUGIN_INSTALL_DIR=~/.local/kde/lib64/qt5/plugins -DQML_INSTALL_DIR=~/.local/kde/lib64/qml
make -j8 install
source prefix.sh
bin/igo

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