Update enabled state of menu/buttons dynamically

[scottlerch]

Enable and disable buttons and menu dynamically depending on current state. For example, only enable Cut, Copy, Paste, and Delete if rows or cells are currently selected.

[WhiteHsu]

@scottlerch I'm interested in this enhancement and would like to give this a try. So may I pick up this item to implement? Thank you. :-)

[scottlerch]

@WhiteHsu sure, if you submit a pull request I'll test it out and if everything looks good I'll create a new release.

[WhiteHsu]

@scottlerch Thanks for reply. I'll give it a try and keep you posted. :-)