Don't allow pasting into non-editable views

My assumption is that pressing command-V on a non-editable view is always a
mistake; it would execute arbitrary commands depending on what's in the
clipboard, so prevent it.

Author: stefanhaller

Diff for: gui.go

@@ -1489,6 +1489,10 @@ func (g *Gui) execKeybindings(v *View, ev *GocuiEvent) error {
 	var globalKb *keybinding
 	var matchingParentViewKb *keybinding
 
+	if g.IsPasting && v != nil && !v.Editable {
+		return nil
+	}
+
 	// if we're searching, and we've hit n/N/Esc, we ignore the default keybinding
 	if v != nil && v.IsSearching() && ev.Mod == ModNone {
 		if eventMatchesKey(ev, g.NextSearchMatchKey) {