Ignore keypresses during IME composition (#1573)

Some IMEs continue sending keypresses even during composition, so we 
just ignore them.

data/core/init.lua

@@ -1278,6 +1278,9 @@ function core.on_event(type, ...)
   elseif type == "textediting" then
     ime.on_text_editing(...)
   elseif type == "keypressed" then
+    -- In some cases during IME composition input is still sent to us
+    -- so we just ignore it.
+    if ime.editing then return false end
     did_keymap = keymap.on_key_pressed(...)
   elseif type == "keyreleased" then
     keymap.on_key_released(...)

data/core/keymap.lua

@@ -203,10 +203,6 @@ end
 -- Events listening
 --------------------------------------------------------------------------------
 function keymap.on_key_pressed(k, ...)
-  -- In MacOS and Windows during IME composition input is still sent to us
-  -- so we just ignore it
-  if PLATFORM ~= "Linux" and ime.editing then return false end
-
   local mk = modkey_map[k]
   if mk then
     keymap.modkeys[mk] = true