Revamp flash_ui handling, once more, with feeling ;) (#7262)

* Simplify flash_ui handling (by handling the unhighlight pre-callback, c.f., #7262 for more details).
* UIManager: Handle translucent window-level widgets (and those wrapped in a translucent MovableContainer) properly in setDirty directly, making sure what's *underneath* them gets repainted to avoid alpha layering glitches. (This was previously handled via localized hacks).
* Update UIManager's documentation, and format it properly for ldoc parsing, making the HTML docs more useful.
* ReaderView: Reinitialize the various page areas when opening a new document, to prevent poisoning from the previous document.
* Event: Handle nils in an event's arguments.
* CheckButton/RadioButton: Switch to simple inversion to handle highlighting
* CheckButton: Make the highlight span the inner frame's width, instead of just the text's width, if possible.
* AlphaContainer: Fix & simplify, given the UIManager alpha handling.
* MovableContainer: When translucent, cache the canvas bb used for composition.
* Avoid spurious refreshes in a few widgets using various dummy *TextWidgets in order to first compute a text height.
* KeyValuePage: Avoid floats in size computations.

frontend/ui/widget/spinwidget.lua

@@ -234,12 +234,9 @@ function SpinWidget:update()
         },
         self.movable,
     }
-    -- If we're translucent, Button itself will handle that post-callback, in order to preserve alpha without flickering.
-    if not self.movable.alpha then
-        UIManager:setDirty(self, function()
-            return "ui", self.spin_frame.dimen
-        end)
-    end
+    UIManager:setDirty(self, function()
+        return "ui", self.spin_frame.dimen
+    end)
 end
 
 function SpinWidget:hasMoved()