Doc fix

Thanks to Ting-Yu Lin for the pointer.
Fix #13: set tags-loop-scan and tags-loop-operate instead
Work seamlessly with exuberant-ctags
Fix #10: Move forward/backward in the navigation history

Consolidate miscellaneous commands into
ggtags-mode-prefix-key (customisable).
Customise ggtags-mode-prefix-key should just work
Add menu-bar entries and release v0.6.8
Document how to make ggtags work with the ctags backend
Tweak ggtags-mode-prefix-map to reduce conflicts

since C-c <char> are popular key bindings by emacs users.
Fix buglets
New navigation commands to move to last/first error
New function ggtags-process-string and use it
Refactor out ggtags-global-start for re-use

Rename ggtags-global-options to ggtags-global-build-command.
Rename ggtags-list-tags to ggtags-find-tag-regexp.
Remove FMT arg to ggtags-global-build-command

it must be globally set to work properly.
Don't save point to the tag marker ring if zero matches

Thus running it in next-error-hook.
More refactoring to add more commands
New helper function ggtags-read-string and use it
New variable ggtags-global-ignore-case
Make it possible to force finding definitions
Doc fix
Fix "Don't save point to the tag marker ring if zero matches"

of commit "7e700e9430a811228e238bffd7d9799cafb5cf5f".