From bug 145732:
"Maybe we could update the fdo cut+paste spec to say that Shift+Middle or
Ctrl+Middle means 'replace the text under the cursor with the selection
buffer'? (And get gtk, qt, and moz to implement that.)"

I'm proposing that shift+middle-click should clear an entry or text view and
insert the text in the "mouse clipboard" (I think it is the secondary
clipboard). ctrl+middle-click should IMHO clear an entry or text view, paste the
contents of the secondary clipboard and activate the text widget, if it is a
GtkEntry. Also, shift-ctrl-p should in my opinion clear the current text widget
and afterwards paste the clipboard contents (I think it is the primary one).