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

!1270
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  Build
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00:03:02

passed build-without-native-backend-and-wayland #771488

00:01:50

passed build-without-opengl-and-glx #771487

00:02:40

 
  Test
passed can-build-gnome-shell #771490

00:03:59

passed test-mutter #771489

00:04:53