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/configdialog.lua

@@ -865,10 +865,15 @@ end
 function ConfigDialog:onShowConfigPanel(index)
     self.panel_index = index
     local old_dimen = self.dialog_frame.dimen and self.dialog_frame.dimen:copy()
+    local old_layout_h = self.layout and #self.layout
     self:update()
     -- NOTE: Keep that one as UI to avoid delay when both this and the topmenu are shown.
     --       Plus, this is also called for each tab anyway, so that wouldn't have been great.
-    UIManager:setDirty(self.is_fresh and self or "all", function()
+    -- NOTE: And we also only need to repaint what's behind us when switching to a smaller dialog...
+    --       This is trickier than in touchmenu, because dimen appear to fluctuate before/after painting...
+    --       So we've settled instead for the amount of lines in the panel, as line-height is constant.
+    local keep_bg = old_layout_h and #self.layout >= old_layout_h
+    UIManager:setDirty((self.is_fresh or keep_bg) and self or "all", function()
         local refresh_dimen =
             old_dimen and old_dimen:combine(self.dialog_frame.dimen)
             or self.dialog_frame.dimen