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/apps/reader/modules/readerbookmark.lua

@@ -423,13 +423,8 @@ function ReaderBookmark:onShowBookmark()
                             UIManager:close(self.textviewer)
                         end,
                     },
-                }
-            },
-            -- Request a full-screen refresh on close, to clear potential flash_ui highlights
-            close_callback = function()
-                -- TextViewer does a "partial" on CloseWidget
-                UIManager:setDirty(nil, "partial")
-            end,
+                },
+            }
         }
         UIManager:show(self.textviewer)
         return true