[IMAPx] Keep store summary's flags in sync with mailbox flags

This way mailbox attributes like \NoSelect gets in sync with
folder info flags, which makes it work properly, the non-selectable
folders will not be updated (tested with GMail's "[GMail]" folder).
[IMAPx] Use case-insensitive compares and hash table functions

Almost all attributes, responses and commands in general are
case-insensitive in IMAPx, thus even the code can "normalize"
the respective strings to proper case, it's safer to use
case-insensitive compares and equal-functions in hash tables
where applicable.