Removed the following menu items under View:
Hide Selected Messages
Hide Read Messages
Show Hidden Messages
These options are confusing and don't fit with the rest of the mailer
design.
Hidden messages were tracked by folder in:
~/.evolution/mail/config/hidestate-<<folder-uri>>
So by simply not loading those files any more we reveal all previously
hidden messages and avoid any risk of lost messages.
Also inverted "Hide Deleted Messages" to "Show Deleted Messages".