Adds Run icon in gutter next to ExUnit tests

[niknetniko]

• This PR adds the "run" icons in the gutter for tests:
  

• There are also some small clean-ups in related code.

For the run icons:

• They use the existing run actions, same as when you right-click.
• They only appear of the file is a test file (suffix _test.exs and in a map marked as tests in the project structure)
• I have no idea how one should write a test for this stuff, so I didn't (I'm fairly new to Intellij SDK stuff)
• I hardcoded "test", "describe", "doctest" and "defmodule". I don't know if it would be better to dynamically determine these, but that would be fairly hard I think.

I have been using this some time in a personal fork and it seems to work pretty well.

Fixes #688.

[KronicDeth]

I have no idea how one should write a test for this stuff, so I didn't (I'm fairly new to Intellij SDK stuff)

Yeah, I usually don't know how to test most of the stuff in the UI. JetBrains has told me before that they don't really write UI tests, so I'm not sure how to test this. Some of the UI stuff ends up being is a line marker attached to the correct line.