Integrate eldoc with gtags

[tuhdo]

Currently I still have to use ggtags because of its Eldoc integration. Combined with ctags backend for other language, I get a pretty good environment for many languages. ggtags uses ggtags-eldoc-function, so maybe it's a good place to start if you want to add this feature. If it can be added, I can finally switch to helm-gtags completely.

[syohex]

I suppose it is better to separate el-doc feature from ggtags, and create new package like gtags-eldoc.

[xin-jin]

I also would like to see this feature. The ggtags-show-definition function is very handy. Any way to just implement this function?