adjusted the menu position of account popup and toolbar arrow.
src/folderview.c: enabled menu popup with Menu key.
use image menu item for part menu.
added toolbar style switch to the context menu of the toolbar.
fixed account selector of compose window having unintended mnemonic.
perform show/hide on trayicon click. Implemented popup menu on trayicon.
Initial import of Sylpheed (GTK2 version).