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/inputdialog.lua

@@ -319,6 +319,7 @@ function InputDialog:init()
             lang = self.lang, -- these might influence height
             para_direction_rtl = self.para_direction_rtl,
             auto_para_direction = self.auto_para_direction,
+            for_measurement_only = true, -- flag it as a dummy, so it won't trigger any bogus repaint/refresh...
         }
         local text_height = input_widget:getTextHeight()
         local line_height = input_widget:getLineHeight()