Wait with screen unload until next screen is loaded and ready to be shown (#74)

* Removed screen flicker when transitioning between screens

* Moved advanced example to subfolder. Added basic example.

* Remove flicker on back navigation too

* Fix issue with no_stack screens added at any time

test/cowait.lua

@@ -4,7 +4,10 @@ function M.seconds(amount)
 	local co = coroutine.running()
 	assert(co, "You must run this from within a coroutine")
 	timer.delay(amount, false, function()
-		coroutine.resume(co)
+		local ok, err = coroutine.resume(co)
+		if not ok then
+			print(err)
+		end
 	end)
 	coroutine.yield()
 end
@@ -13,10 +16,13 @@ function M.eval(fn, timeout)
 	local co = coroutine.running()
 	assert(co, "You must run this from within a coroutine")
 	local start = socket.gettime()
-	timer.delay(0.01, true, function(self, handle, time_elapsed)
+	timer.delay(0.02, true, function(self, handle, time_elapsed)
 		if fn() or (timeout and socket.gettime() > (start + timeout)) then
 			timer.cancel(handle)
-			coroutine.resume(co)
+			local ok, err = coroutine.resume(co)
+			if not ok then
+				print(err)
+			end
 		end
 	end)
 	coroutine.yield()