Add configurable key bindings for selecting next/previous hotbar item and changing the volume

This PR adds configurable key bindings for:

* selecting next item in hotbar (default: `N`)
* selecting previous item in hotbar (default: `B`)
* increasing volume by 10% (default: `.`)
* decreasing volume by 10% (default: `,`)
* muting (set volume to 0%) (default: `M`)

The key bindings for hotbar item selection are additional to the number keys and the mouse wheel key binding; they do not replace them.

New `keymap_` settings are provided. Also the keybinding menu contains the new key bindings.