app: Improve --project handling

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