Open in Terminal from Files does not focus Terminal

Move this issue from https://github.com/elementary/files/issues/553#issuecomment-405961781

elementary: 5.0
Files: 0.3.5
Gala: 0.3.1
Terminal: 0.5.0

### Reproduce
1. Open a Terminal
2. Go to a new workspace
3. Open another Terminal
4. Go to a new workspace
3. Open Files
4. From Files context menu try to open a new Terminal

### Outcome
A new tab opens in one of the two running terminals and Plank gets a red indicator that is btw hardly noticeable. Clicking it will move you to the workspace on this Terminal

### Expected Behavior
When a Terminal doesn't exist on the current workspace, then open a new Terminal. I believe this is the best design approach for workspaces use. You expect a (new) Terminal on the current working workspace

Note that if a Terminal already exists on the current workspace it will bring it into focus

Also see: https://github.com/elementary/files/issues/99