deletemessageheaders(): use passed list of headers to remove

... and not self.filterheaders.  With the current code this change
is no-op (since self.filterheaders is always passed as header_list),
but it is a bug in general.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Class IMAPServer: idle() is private now

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Class LocalStatusRepository: add missing self qualifier

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Class LocalStatusFolder: remove duplicate cachemessagelist()

Remained from GMail label sync merge.

Pointyhat-to: Eygene Ryabinkin
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Add import for OfflineImapError

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Fix multiple typos in var, function and exception names

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>