Split `keyboard` `xmodmap_fixes` into smaller fn's
Split `keyboard` `xmodmap_fixes` into smaller fn's

Makes it much clearer what each part is doing:

- `xmodmap_files` runs `xmodmap` on an `.xmodmap` file if it exists
- `system_key` runs `macbook_mousekeys` on a MacBook, else `windows_key`
- `macbook_mousekeys` sets up some better buttons for mousekeys
- `windows_key` sets up `Super_L` to be either `mod4` or `mod3`
- `alt_is_meta` sets right-`<Alt>` to be a `<Meta>` key

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