UI Behavior tweaks (#3983)

* Switch all initial highlights to "fast" update

i.e., everything that does an invert
Plus a few other things that refresh small UI elements onTap
Re #3130

* Tweak refreshtype for a number of widgets:
  * Fix iconbutton dimen
  * Make touchmenu flash on close & initial menu popup. Full-screen on close.
  * Use flashing updates when opening/closing dictionary popup. Full-screen on close.
  * Switch FileManager to partial.
    It's mostly text, and we want flash promotion there.
  * Make configdialog & menu flash on exit
  * Make FLWidget flash on close
  * virtualkeyboard: flash on layout change & popup.
  * Potentially not that great workaround to ensure we actually see the
highlights in the FM's chevrons
  * Flash when closing BookStatus Widget
  * Optimize away a quirk of the dual "fast" update in touchmenu

* Promote updates to flashing slightly more agressively.

* Document what each refreshtype actually does.

With a few guidelines on their optimal usecases.

* Switch remaining scheduleIn(0.0) to nextTick()

* Tighter scheduling timers

Shaving a hundred ms off UI callbacks...

* Cache FFI C Library namespace

* Ask MuPDF to convert pixmaps to BGR on Kobo

Fix #3949

* Mention koxtoolchain in the README

re #3972

* Kindle: Handle *all* fonts via EXT_FONT_DIR instead of bind mounts insanity

* Make black flashes in UI elements user-configurable

(All or nothing).

* Jot down some random KOA2 sysfs path

frontend/document/credocument.lua

@@ -7,6 +7,7 @@ local Geom = require("ui/geometry")
 local RenderImage = require("ui/renderimage")
 local Screen = require("device").screen
 local ffi = require("ffi")
+local C = ffi.C
 local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")
 
@@ -205,7 +206,7 @@ function CreDocument:getCoverPageImage()
     local data, size = self._document:getCoverPageImageData()
     if data and size then
         local image = RenderImage:renderImageData(data, size)
-        ffi.C.free(data) -- free the userdata we got from crengine
+        C.free(data) -- free the userdata we got from crengine
         return image
     end
 end
@@ -215,7 +216,7 @@ function CreDocument:getImageFromPosition(pos, want_frames)
     if data and size then
         logger.dbg("CreDocument: got image data from position", data, size)
         local image = RenderImage:renderImageData(data, size, want_frames)
-        ffi.C.free(data) -- free the userdata we got from crengine
+        C.free(data) -- free the userdata we got from crengine
         return image
     end
 end