make window manager selectable from lumina-config>settings>general
minor fix
note user about consequences of not using fluxbox
partially revert commit b4803118c5e22e7a00d313698868a0ebbe5188ba, making default-windowmanager an cli-only option by not exposing a UI
Merge pull request #15 from Nanolx/master

make window manager selectable from lumina-config>settings>general