Fix #36: Allow customising path to global's exectuables
Use read-only-mode in ggtags-toggle-project-read-only
Teach ggtags-find-tag-dwim to detect #include files

New user variable ggtags-include-pattern.
Add new command ggtags-save-to-register for #37
New command ggtags-navigation-isearch-forward
Simplify ggtags-save-to-register by always re-do the search

Fix error in ggtags-global--display-buffer.
Fix thinko in "Don't display buffer if ggtags-navigation-mode is exited"

of commit a4ba72b12124d125e49db217603c6542fbc98a5b.

Fix ggtags-handle-single-match not to display the window if
interrupted.