undo/redo/paste toolbar buttons are never grayed out

There are three ways to use undo/redo/paste:

* Right click on a pad -> select Edit -> select Undo/Redo/Paste: there these menu items can be grayed out when there is nothing to undo/redo/paste
* From a toolbar: there undo/redo/paste are never grayed out (while cut/paste are when nothing is selected, so it is inconsistent)
* Using a keyboard shortcut: nothing to gray out here so it is not in the scope of this bug

It is possible to gray things out in the toolbar (for example the cut/copy buttons are disabled unless some text is selected and also XpadTextBuffer have a pointer Xpad in XpadTextBufferPrivate so it can mess with toolbar for undo/redo at least).

We should do the same for undo/redo/paste buttons on a toolbar to improve look-and-feel. undo/redo/paste buttons should be grayed out under same condition as when they are grayed out in the menu.