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.

spec/unit/readerfooter_spec.lua

@@ -270,6 +270,9 @@ describe("Readerfooter module", function()
         assert.is.same(1, footer.mode)
         footer:onTapFooter()
         assert.is.same(0, footer.mode)
+        -- Make it visible again to make the following tests behave...
+        footer:onTapFooter()
+        assert.is.same(1, footer.mode)
     end)
 
     it("should pick up screen resize in resetLayout", function()