get/put text from the source application to/from macvim

For me, there is no use for refocus.scpt since when I close MacVim, I get already to the previous app.
Using this information, I copy the text of the currently active text box to use as basis for MacVim, and when I finish, I copy back.

Haven't tried this out for that many apps, in some cases "select-all" might not be a very good option, but I don't have any other idea…

Moreover "System Events" requires accessibility features to be activated…

BTW, removed a [useless use of cat](https://en.wikipedia.org/wiki/Cat_(Unix)#Useless_use_of_cat)