added new plug-in signals for preferences dialogs (#30)
added UI for SOCKS settings.
added new account option 'Put signature before quote (not recommended)'.
removed warnings 'GtkSpinButton: setting an adjustment with non-zero page size is deprecated'.
spaces in address and server entries are automatically removed at applying account configuration.
automatically set SSL ports in Advanced tab when the radio buttons in SSL tab are toggled.
made labels more descriptive.
added the option 'Filter new messages in INBOX on receiving'.
don't make GtkTextView auto-expanded in prefs dialogs.
implemented auto-input of signature on new account setting.
added the option to input signature directly.
modified the description of 'Download all messages' option.
disable POP before SMTP option with non-POP3 account.
impelmented POP before SMTP.
supported PLAIN authentication on IMAP4.
save temporary IMAP4 password. Don't try to connect to IMAP4 server when closing folder.
implemented clearing of IMAP4 message caches on exit.
added an option to update only INBOX on checking new messages of IMAP4 accounts.
enabled custom queue folder.
popup main window when account edit dialog is closed.
fixed focus problem in win32.
separated UI code from prefs_common.c and prefs_account.c.