String file housekeeping

[phet-steele]

Looking at masses-and-springs-strings_en.json, there are some small changes to be made to follow convention and to clean it up:

• I would recommend making this file alphabetical.
• Screen title string keys should follow the convention: screen.{{screenName}}, eg.

 "screen.intro": {
    "value": "Intro"
  }

not

 "intro": {
    "value": "Intro"
  }

• Vector (the screen) should be renamed to Vectors as per request
• Should remove "0 g" (see Remove "0g" as a preset gravity option #159):

 "body.zeroG": {
    "value": "0 g"
  }

• "Slow Motion" should be changed to "Slow". Change the string key as well.

For phetsims/qa/issues/67.

[phet-steele]

Looks good to me, thanks @Denz1994. We'll close this and wait for a code review to see if there are any remaining issues. Note that also now all files that still say "Vector", like VectorScreenView.js, should be changed to "Vectors". Ought to be fun.

[Denz1994]

The variable and file names that reference the Vectors tab have been updated.