Fix #11: Right click support.

When clicked, Button() and Input() return the respective mouse button
constant ('l', 'r', 'm') instead of just true.
When activated by keyboard, Button() and Input() return 'return'.
Otherwise Button() and Input() return false.

keyboard.lua

@@ -70,6 +70,10 @@ local function makeCyclable(id)
 	lastwidget = id
 end
 
+local function pressedOn(id, k)
+	return (k or 'return') == key and hasFocus(id) and k
+end
+
 local function beginFrame()
 	-- for future use?
 end
@@ -88,6 +92,7 @@ return setmetatable({
 	clearFocus    = clearFocus,
 	hasFocus      = hasFocus,
 	makeCyclable  = makeCyclable,
+	pressedOn     = pressedOn,
 
 	disable       = disable,
 	enable        = clearFocus,