Preprocess the Clipboard content before a paste 

This commit introduce a preprocessing of the Clipboard so that wikitext 
can adapt the clipboard content to the target markup text.

An example of adaptation is included with this commit :
 - if there is an image in the clipboard, a png file is generated
 - the Clipboard is extended with the markup referencing the png file.
Auto-conversion to markup when pasting html
Fix potential NPE if a Link Span has no href
Add resource dialog to choose where the image will be stored.

This dialog can guess the name of file using a text selection.
This dialog remember last choice depending of the edited file.