Fix confusion with paste buffers and keyboard shortcuts

[andydotxyz]

Fixes #134

[drunlade]

I haven't poked into all the code to check, but it seems unnecessary to pass the clipboard in here when you could access it directly via fyne.CurrentApp().Clipboard() instead and simplify the calling code.

Suggested change

	func (t *Terminal) pasteText(clipboard fyne.Clipboard, secondary bool) {
	var content string
	if secondary {
	content = t.SelectedText()
	} else {
	content = clipboard.Content()
	}
	func (t *Terminal) pasteText(secondary bool) {
	// Not sure if it's possible for Clipboard to be nil.
	// I assume fyne.CurrentApp() will never be nil.
	if fyne.CurrentApp().Clipboard() == nil {
	return
	}
	var content string
	if secondary {
	content = t.SelectedText()
	} else {
	content = fyne.CurrentApp().Clipboard().Content()
	}

[andydotxyz]

Thanks for the suggestion, but I saw a cleaner solution - eliminating the bool and actually having two clipboards that model the two different ways of copy/paste.

The benefit of this is that selected text is remembered for later instead of only being available whilst currently selected.