New command ggtags-show-definition

New function ggtags-global-output to handle global output.
Rename ggtags-global-next-error-hook to ggtags-find-tag-hook.
Give ggtags-show-definition-function full information

of the definition candidates.
Don't display buffer if ggtags-navigation-mode is exited

or we will undo the work of ggtags-navigation-mode-done.
Combine ggtags-global-column and tag name search

Cannot rely on ggtags-global-column entirely because the source text
may or may not include the whole source line depending on plugin and
output format. See discussion
http://thread.gmane.org/gmane.comp.gnu.global.bugs/1675