Make seed uncopyable

Other wallets, such as Mycelium, do not allow the user to simply copy
and paste the seed. This is very useful to assure users follow
directions on the next screen, because previously it was easily possible
to just copy the seed and paste it on the next screen. The user's wallet
would work well for a while like this, but without having the seed
written down it's more of a ticking time bomb than a wallet.

I recommend pulling this patch as I have read many cases where users do
not write the seed down, and I believe that the main cause is lack of
friction forcing them to do so.

This patch was inspired by reading this Reddit conversation:

https://www.reddit.com/r/Bitcoin/comments/3p4bq1/electrum_v25/cw380kg

But I'm not a participant in that convo.

gui/qt/seed_dialog.py

@@ -75,7 +75,7 @@ def enter_seed_box(msg, window, sid=None, text=None):
         seed_e = ScanQRTextEdit()
         seed_e.setTabChangesFocus(True)
     else:
-        seed_e = ShowQRTextEdit(text=text)
+        seed_e = ShowQRTextEdit(text=text, paranoid=True)
     seed_e.setMaximumHeight(130)
     vbox.addWidget(label)
     grid = QGridLayout()