Remove old clipboard workarounds after changes in GLFW

See fyne-io#1136. The clipboard no longer crashes when pasting empy content (also verified locally). The clipboard also no longer require access to the window struct. Clean up the code. As the clipboard no longer needs the viewport, we can also remove a possible crash when getting a clipboard from a window that has yet to be set up.

internal/driver/glfw/clipboard.go

@@ -16,9 +16,7 @@ import (
 var _ fyne.Clipboard = (*clipboard)(nil)
 
 // clipboard represents the system clipboard
-type clipboard struct {
-	window *glfw.Window
-}
+type clipboard struct{}
 
 // Content returns the clipboard content
 func (c *clipboard) Content() string {
@@ -64,12 +62,6 @@ func (c *clipboard) SetContent(content string) {
 
 func (c *clipboard) setContent(content string) {
 	runOnMain(func() {
-		defer func() {
-			if r := recover(); r != nil {
-				fyne.LogError("GLFW clipboard error (details above)", nil)
-			}
-		}()
-
 		glfw.SetClipboardString(content)
 	})
 }

internal/driver/glfw/window.go

@@ -229,14 +229,7 @@ func (w *window) ShowAndRun() {
 
 // Clipboard returns the system clipboard
 func (w *window) Clipboard() fyne.Clipboard {
-	if w.view() == nil {
-		return nil
-	}
-
-	if w.clipboard == nil {
-		w.clipboard = &clipboard{window: w.viewport}
-	}
-	return w.clipboard
+	return &clipboard{}
 }
 
 func (w *window) Content() fyne.CanvasObject {

internal/driver/glfw/window_desktop.go

@@ -77,8 +77,6 @@ type window struct {
 	icon         fyne.Resource
 	mainmenu     *fyne.MainMenu
 
-	clipboard fyne.Clipboard
-
 	master     bool
 	fullScreen bool
 	centered   bool