Dict, Trapper: prevent dismissal by past events

Add UIManager:discardEvents(), to allow dropping events
for a period of time.
Default duration of 600ms on eInk, 300ms otherwise.
Used when showing the dict result window, so that a tap
happening just when it's being shown won't discard it.
Used with Trapper:confirm()/:info(), to avoid taps made
in a previous processing to dismiss (and so, select the
cancel action) a ConfirmBox not yet painted/visible.

frontend/apps/reader/modules/readerdictionary.lua

@@ -889,6 +889,12 @@ function ReaderDictionary:showDict(word, results, box, link)
     self:dismissLookupInfo()
     if results and results[1] then
         UIManager:show(self.dict_window)
+        if not results[1].lookup_cancelled then
+            -- Discard queued and coming up events to avoid accidental
+            -- dismissal (but not if lookup interrupted, so 2 quick
+            -- taps can discard everything)
+            UIManager:discardEvents(true)
+        end
     end
 end