Fix scrolling, add inertial scroll on non-eInk devices

Add a new reader module: ReaderScrolling, that exposes
some Scrolling options to the menu (which are to be used
by and implemented in ReaderPaging and ReaderRolling
themselves) and implement some inertial scrolling logic
used by both of them.
Default to "Classic scrolling" which is the expected
behaviour on phones and tablets.
The old CreDocument buggy behaviour is available as
"Turbo scrolling" for both Paging and Rolling documents.
Added a "On release scrolling" option that might be
useful on eInk to avoid dynamic pan/scrolling.

Try to avoid bad interactions between pan and swipe,
cancelling unwanted panning if we ended up doing a
swipe or multiswipe.

frontend/apps/reader/modules/readerview.lua

@@ -73,6 +73,9 @@ local ReaderView = OverlapGroup:extend{
     flipping_visible = false,
     -- to ensure periodic flush of settings
     settings_last_save_tv = nil,
+    -- might be directly updated by readerpaging/readerrolling when
+    -- they handle some panning/scrolling, to request "fast" refreshes
+    currently_scrolling = false,
 }
 
 function ReaderView:init()
@@ -614,7 +617,7 @@ function ReaderView:recalculate()
     end
     -- Flag a repaint so self:paintTo will be called
     -- NOTE: This is also unfortunately called during panning, essentially making sure we'll never be using "fast" for pans ;).
-    UIManager:setDirty(self.dialog, "partial")
+    UIManager:setDirty(self.dialog, self.currently_scrolling and "fast" or "partial")
 end
 
 function ReaderView:PanningUpdate(dx, dy)