Update - Clipboard Paste Function Correction (BookView editor only)

This correction corrects following:

1) CTRL+V (Command+V in MacOS X) and SHIFT-Insert keyboard shortcuts are now hooked to the code of BookViewEditor.cpp - this allows the Undo function to work properly (before the CTRL+V shortcut Undo didn't work, context menu Paste function worked with Undo) and also it allows to overrides the default Clipboard paste function behavior consistently (see point 2)

2) New code is added to the BookViewEditor.cpp function BookViewEditor::paste() to allow end-user driven decision whether 
the content of the clipboard (if HTML formatted) should be pasted as text without the original formatting (using page()->triggerAction(QWebPage::PasteAndMatchStyle)) or whether the original HTML formatting from Clipboard should be used. 

Some details about the bug observation and reasoning why this behavior is needed - see http://code.google.com/p/sigil/issues/detail?id=2423.