Added “Paste and Keep Data” feature to draw tools plugin

When I'm exchanging draw tools data with others, I sometimes want to combine my drawing with theirs to make one unified drawing. This change allows users to do exactly that.

This change adds an additional button to the "Drawtools Opt" options, labeled "Paste and Keep Data". It behaves the same way as "Paste Drawn Items", except that it also merges the existing draw data with the pasted data using the jQuery method $.merge().