menu: allow overwriting submenu icon

[tokyo4j]

This PR allows overwriting the icon of item linking to another menu like below (the icon for krita instead of mpv should be shown):

<openbox_menu>
  <menu id="static-menu" label="Static Menu" icon="mpv" />
  <menu id="root-menu" label="Root">
    <menu id="static-menu" icon="krita" />
  </menu>
</openbox_menu>

For comparison, Openbox seems to ignore icon property in toplevel menu definition in the first place and always follows icon property inside the <menu> that links to a static menu. So when we have following menu.xml:

<openbox_menu>
  <menu id="static-menu" label="Static Menu"
    icon="/usr/share/icons/hicolor/32x32/apps/mpv.png" />
  <menu id="root-menu" label="Root">
    <menu id="static-menu"
      icon="/usr/share/icons/hicolor/22x22/apps/krita.png" />
    <menu id="inline-menu" label="Inline Menu"
      icon="/usr/share/icons/hicolor/32x32/apps/chromium.png" />
  </menu>
</openbox_menu>

Openbox	labwc 0.9.0	This PR

So I think this PR enhances compatibility to Openbox.

This PR also fixes my mistake in #2971 (s/parent->icon/menu->icon/) that showed incorrect icon in an item linking to another menu.

Link: #2992 (reply in thread)

[johanmalm]

Thanks for analysis and fix. Makes sense.

LGTM