Combobutton is not tab focusable if no tabIndex or tabindex="0"

If a combobutton does not receive a tabindex, or if tabindex is set to 0 in the zul file, the combobutton does not receive a tabindex.
This is normal for other controls, but as tabindex is rendered by <span> elements that are not automaticaly focusable (button, etc don't require a tabindex to be focusable)
This cause the combobuttons to not be focusable unless a tabindex > 0 is set.

Additionaly, when tab focused, the styling is not applied to the correct span, which causes the :focus style to not be applied.

reproduce with:

<zk>
<window border="normal" title="hello">

<div>
Navigate between page components using TAB key, Combobutton
component fail to obtain Focus
</div>

<hbox hflex="1">
<button label="Button" focus="true" />
<combobutton label="ComboButton" tabindex="0">
<menupopup>
<menuitem label="Item 1" />
</menupopup>
</combobutton>
<combobutton label="ComboButton" tabindex="1">
<menupopup>
<menuitem label="Item 1" />
</menupopup>
</combobutton>
</hbox>

</window>
</zk>