some helpers

I spent some time trying to reduce the duplication between identical
operations with and without the shift key pressed. However it makes
things harder to understand. If you try to process selection in
App.keychord_pressed in main.lua, you have to sometimes process the
selection before (e.g. including the state of the cursor _before_ an
arrow key takes effect), and sometimes after (e.g. copying the selection
before resetting it, even though it doesn't include a shift key)

Let's just leave things as they are.

keychord.lua

@@ -33,6 +33,22 @@ function App.modifier_down()
   return array.any(Modifiers, love.keyboard.isDown)
 end
 
+function App.ctrl_down()
+  return love.keyboard.isDown('lctrl') or love.keyboard.isDown('rctrl')
+end
+
+function App.alt_down()
+  return love.keyboard.isDown('lalt') or love.keyboard.isDown('ralt')
+end
+
+function App.shift_down()
+  return love.keyboard.isDown('lshift') or love.keyboard.isDown('rshift')
+end
+
+function App.gui_down()
+  return love.keyboard.isDown('lgui') or love.keyboard.isDown('rgui')
+end
+
 array = {}
 
 function array.find(arr, elem)