ui: Anchor the Observers icon to the sidebar, going right to left

Supersedes 9e0593d714, which attempted to
fix the issue with the icon displaying at unexpected places by anchoring
it to the battery display from the left instead of staying true to the
intended design.

CC #5575

data/themes/default.cfg

@@ -364,8 +364,8 @@
             [observers]
                 id=observers
                 font_size={DEFAULT_FONT_SMALL}
-                ref=battery-box-topright
-                rect="+5,=,+16,+{DEFAULT_FONT_SMALL_HEIGHT}"
+                ref=sidebar-panel
+                rect="=-22,=+4,+16,+16" # x = 16 + 6
                 xanchor=right
                 yanchor=fixed
             [/observers]