add another api callback to determine whether menu items should appear
add another api callback to determine whether menu items should appear

this way we can e.g. not show the git submenu unless a git repo is
actually present

nywilken/dotfiles,tschundler/dotfiles,nywilken/dotfiles,iff/dotfiles-old,iff/dotfiles-old,tschundler/dotfiles,tschundler/dotfiles