Add context menu to wxGuruSpeaksPanel

galaxql.cpp
galaxql.h

- Add a context menu to the lessons panel aka wxGuruSpeaksPanel.

(wxGuruSpeaksPanel::OnContextMenu)
(wxGuruSpeaksPanel::OnContextMenu_Copy)
(wxGuruSpeaksPanel::OnContextMenu_SelectAll)
- Add the ability to copy and select all text via right-click.


This commit closes a sourceforge ticket:
"Add context menu for copy to Guru's window"
http://sourceforge.net/p/galaxql/tickets/10/