Add more generic ground symbols

[evanshultz]

Often there may multiple grounds separated just for noise reasons. There could be a few ohms or a bead between them, for example. They aren't specifically GNDA or GNDD or GNDS, just a partition of a common ground. So here are a three symbols to assist with that.

---

All contributions to the kicad library must follow the KiCad library convention

Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:

• Provide a URL to a datasheet for the symbol(s) you are contributing
• An example screenshot image is very helpful
• Ensure that the associated footprints match the official footprint library
  • A new fitting footprint must be submitted if the library does not yet contain one.
• If there are matching footprint PRs, provide link(s) as appropriate
• Check the output of the Travis automated check scripts - fix any errors as required
• Give a reason behind any intentional library convention rule violation.

[cpresser]

Those 3 `~' were the only ones left in power.lib
There are a few more in Graphics.lib. And I confirmed that saving with 2.4 format removes them.