move "show syntax groups" keybind

I tend to search for this in vimrc-keys anyway, so it might as well be there.
enable "show syntax groups" bind and set to F9
use F3 for (currently disabled) xmp bind
disable most F-key binds in insert mode

F10 (toggle keymap) has been kept, since it's directly applicable to insert
mode.
change yellowish Normal color to light gray