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.

mouse.lua

@@ -27,7 +27,7 @@ THE SOFTWARE.
 local _M -- holds the module. needed to make widgetHit overridable
 
 local x,y = 0,0
-local down = false
+local down, downLast = false, false
 local hot, active = nil, nil
 local NO_WIDGET = {}
 local function _NOP_() end
@@ -55,13 +55,17 @@ local function updateWidget(id, pos, size, hit)
 end
 
 local function releasedOn(id)
-	return not down and isHot(id) and isActive(id)
+	return not down and isHot(id) and isActive(id) and downLast
 end
 
 local function beginFrame()
 	hot = nil
 	x,y = love.mouse.getPosition()
-	down = love.mouse.isDown('l')
+	downLast = down
+	down = false
+	for _,btn in ipairs{'l', 'm', 'r'} do
+		down = down or (love.mouse.isDown(btn) and btn)
+	end
 end
 
 local function endFrame()