Minor Input & TimeVal cleanups

* Input: Don't create a new TimeVal object for input frame timestamps, just promote our existing table by assigning it the `TimeVal` metatable.
* TimeVal: Export (const) `zero` & `huge` TimeVal objects, because they're common enough in our codebase. (NOTE: not actually const, that's a Lua 5.4 feature ;p).
* GestureDetector: Explain the behavior of the `last_tevs` & `first_tevs` tables, and why one needs a new object and not the other.
* Speaking of, simplify the copy method for `first_tevs`, because it doesn't need to create a new TimeVal object, we can just reference the original, it's unique and re-assigned for each frame.

frontend/apps/reader/modules/readerdictionary.lua

@@ -110,7 +110,7 @@ function ReaderDictionary:init()
     -- Allow quick interruption or dismiss of search result window
     -- with tap if done before this delay. After this delay, the
     -- result window is shown and dismiss prevented for a few 100ms
-    self.quick_dismiss_before_delay = TimeVal:new{ sec = 3 }
+    self.quick_dismiss_before_delay = TimeVal:new{ sec = 3, usec = 0 }
 
     -- Gather info about available dictionaries
     if not available_ifos then