FootnoteWidget: Flag as covers_footer

When footnotes are displayed as "popups", they're displayed over ReaderUI
in a BottomContainer, so they pretty much always cover the footer ;).

(i.e., they're really really not popups *at all* :D).

This prevents spurious status bar auto-refreshes.

Re #12323

frontend/ui/widget/footnotewidget.lua

@@ -131,6 +131,7 @@ local FootnoteWidget = InputContainer:extend{
     on_tap_close_callback = nil,
     close_callback = nil,
     dialog = nil,
+    covers_footer = true,
 }
 
 function FootnoteWidget:init()