Themes: Fix the observers icon being hidden under the minimap

Closes #3543. Fixups #3441.

The observers icon is declared immediately after `report_timeout`,
therefore the timeout widget was the ref rect for the observers widget:
83ba09a5a9/src/theme.cpp (L120)

PR #3441 added a battery icon to the left of the timeout widget. That
shifted the timeout widget to the right and the observers widget shifted
along with it due to its relative positioning. The observers widget
moved even on devices that didn't have a battery.

Fix this by using the battery box as the ref of the observers widget,
because it currently occupies the absolute location the observers widget
was positioned relatively to.

(cherry picked from commit 461cd2c4bd)

data/themes/default.cfg

@@ -351,6 +351,7 @@
             [observers]
                 id=observers
                 font_size={DEFAULT_FONT_SMALL}
+                ref=battery-box-topright
                 rect="+5,=,+16,+{DEFAULT_FONT_SMALL_HEIGHT}"
                 xanchor=right
                 yanchor=fixed