[IconMenu] Add ability to style underlying List inside of Menu.

Currently there is no way to style the underlying list when using IconMenu. This change would enable passing a listStyle object similar to how Menu already allows for and just pass the listStyles down to the Menu object.

const listStyles = { paddingTop: 0, paddingBottom: 0 };

<IconMenu listStyle={listStyles} menuStyle={...}>
  <MenuItem .../>
</IconMenu>

[mrprofessor]

Using MenuListProps should work.

<Menu  MenuListProps={{ style: { padding: 0 } }} >
  <MenuItem> First </MenuItem>
  <MenuItem> Second </MenuItem>
  <MenuItem> Third </MenuItem>
</Menu>```

[hbeckeri]

I had a lot of trouble trying to remove the padding: 8px 0 as css classes couldn’t reach the div and the menuStyle, lineSyle props didn’t work either. However wrapping my listing content in a disabled menuItem and setting some negative margins works pretty good.

<IconMenu >
  <MenuItem className="icon-menu-wrapper" disabled={true}>
     <div>listing content</div>
     <div>more listing content</div>
     ...
  </MenuItem>
</IconMenu>

.icon-menu-wraper {
    margin: -8px 0 -8px 0; /* the real magic */
    padding: 0 !important; /* removes left and right padding */
    cursor: initial !important;
}