Make ggtags-create-tags invalidate ggtags-project-root

So that buffers can see the new project root.
Fix #65: Clear ggtags-project-root based on default-directory
Fix #64: Bind M-s s to ggtags-navigation-isearch-forward

Thanks to tuhdo for suggesting.
Fix #60: Declutter command and highlight tag in search history
Refactoring: Use a major mode for viewing search history

Rename command ggtags-global-rerun-search to
ggtags-view-search-history.
Fix #58: Search history can be manipulated with C-k and C-y
Replace kill-buffer-and-window with ggtags-kill-window

because the former throws an error when the window is the only one in
a frame.

Some polish to last change.
Fix ggtags-global-handle-exit to conditionally pop up window

A regression introduced in commit
65aba1399e51e69185b1a8bbf75821a5a7764655

    "For #45: Fix popping up/down window for search with single match"