Support multiple mouse buttons combined

Also remove old comment - that was supposed to reserve the iota, but
it got broken some time ago
Fixes fyne-io#1533

CHANGELOG.md

@@ -21,6 +21,7 @@ More detailed release notes can be found on the [releases page](https://github.c
 * Entry copy/paste is crashing on android 7.1 (#1511)
 * Fyne package creating invalid windows packages (#1521)
 * Menu bar initially doesn't respond to mouse input on macOS (#505) 
+* desktop.MouseHover Button state is not reliable (#1533)
 
 
 ## 1.4 - 1 November 2020

driver/desktop/mouse.go

@@ -7,10 +7,7 @@ type MouseButton int
 
 const (
 	// LeftMouseButton is the most common mouse button - on some systems the only one
-	LeftMouseButton MouseButton = iota + 1
-
-	// TODO: Expose this button
-	// middleMouseButton
+	LeftMouseButton MouseButton = 1 << iota
 
 	// RightMouseButton is the secondary button on most mouse input devices.
 	RightMouseButton

internal/driver/glfw/window.go

@@ -682,9 +682,9 @@ func (w *window) mouseClicked(_ *glfw.Window, btn glfw.MouseButton, action glfw.
 	}
 
 	if action == glfw.Press {
-		w.mouseButton = button
+		w.mouseButton |= button
 	} else if action == glfw.Release {
-		w.mouseButton = 0
+		w.mouseButton &= ^button
 	}
 
 	if wid, ok := co.(fyne.Draggable); ok {