Summary:
Before this patch `--project PROJ` had several flaws:
- Could not be used to open the project in an existing instance
- Could not be used to open a project if there was an already
  running KDevelop instance

With this change:
- We'll check which sessions contain PROJ
  - We'll use the first match as the 'target session' to be opened
- Continue as usual
  - If there's a KDevelop instance with that session open: activate
  - If not: Open a new instance with said session

Reviewers: dfaure, brauch

Reviewed By: brauch

Subscribers: brauch, cullmann, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8842