wayland: Avoid popup grab focus changes if there's an implicit grab
Event delivery with grabbing popups stay essentially the same within the grabbing client, we still must honor the implicit grab as long as there is one.
This is however not the case, the popup_grab_focus() function ignores the button state. The popup_grab_button() function will already indirectly re-sync the focus surface after the last button is released, so checking for button state in popup_grab_focus() is sufficient to make the implicit grab honored with popup grabs involved.
Fixes: #1275 (closed)