Missing mnemonics and capitalization in exta pane related menu entries - freeze breack request

[Luca Ferretti <elle uca libero it>]

It seems the menu entries related to extra pane feature are currently
missing mnemonics and header capitalization here and there.

Here is a preliminary patch here[1] but of course it will need approval
for string freeze break. Could be good fix this issue ASAP.

PS I'm not sure, but it seems that the "switch to other pane" menu entry
doesn't appear anywhere :(


[1] https://bugzilla.gnome.org/show_bug.cgi?id=612021