Add secure "Display entry" to entry menu.

[jsbien]

I think I found a very simple way to solve the problem described in https://bitbucket.org/jsbien/ndt/downloads/djview4poliqarp3.pdf section 2.

Let's take the entry

arwan ※ ob. Porwan

as an example. Let's assume I want to search for 'Porwan' both in the corpus and in the index. Now I have to use 'Edit entry" to copy the word, which is dangerous as may introduce incidental changes to the entry.

[Michał Rudolf]

Add copy actions to popup menu. See #287.

→ <<cset 1b6cb0d58200>>

[jsbien]

OK