Minor state handling tweaks when the footer is disabled/invisible (#5494)

* Don't break `doc_pages` updates when it's completely disabled. (c.f., the commit's comment, doing it on every page turn seems a bit overkill, but I get that it's probably done that way because it was easier ;)).
* Detect the empty footerTextGenerator properly in `_updateFooterText` (it's `""`, not `nil`).
* Abort `_updateFooterText` early when the footer is invisible (and has been for a while/ever, i.e., when not requesting a repaint, like a mode switch would).
* Never send `SetBottomPageMargin` events twice.
* Minor tweaks to touchmenu & configdialog to avoid useless repaints (don't redraw the reader/fm unless we absolutely need to).
* Make sure we show the progress bar alone instead of nothing when disabling the last enabled mode in the menu.

frontend/ui/widget/touchmenu.lua

@@ -614,7 +614,9 @@ function TouchMenu:updateItems()
     -- NOTE: We use a slightly ugly hack to detect a brand new menu vs. a tab switch,
     --       in order to optionally flash on initial menu popup...
     -- NOTE: Also avoid repainting what's underneath us on initial popup.
-    UIManager:setDirty(self.is_fresh and self.show_parent or "all", function()
+    -- NOTE: And we also only need to repaint what's behind us when switching to a smaller menu...
+    local keep_bg = old_dimen and self.dimen.h >= old_dimen.h
+    UIManager:setDirty((self.is_fresh or keep_bg) and self.show_parent or "all", function()
         local refresh_dimen =
             old_dimen and old_dimen:combine(self.dimen)
             or self.dimen