To make encoding selection easier, add a menu that
lists available encodings to the Options window.

Menu structure is borrowed from Firefox.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Tested-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>