[EMACS] turn on which-func-mode

which-func-mode is hooked on prog-mode-hook.
An alternative is global-semantic-stickyfunc-mode,
but it can only show up in header line which is conflict with tabbar

modeline format is updated to make function not as the last item.
[EMACS] enable semantic features
[EMACS] add ggtags hotkeys
[EMACS] automate bookmark+-desktop
(1) add auto-save the last when switching sessions
(2) add auto-save the last when emacs quits
[EMACS] remove redundant custom-set-variables section