Progress on Projects and Sessions

Since the last snapshot I've mostly been working on the support for projects and sessions, because these are the main areas that still need work before we can release Tiled 1.4. There's still a few tasks remaining so I expect there to be one more snapshot release before the Tiled 1.4 Release Candidate.

Changelog

• Project: Added project-specific extensions folder
• Project: Select currently edited file in Project view
• Project: Added 'Open with System Editor' to file context menu
• Project: Show template files in the Project instead of in the Templates view
• Session: Moved the loaded worlds to the session
• Session: Moved the stamps directory to the session
• Session: Moved the "AutoMapping while drawing" option to the session
• Session: Fixed issues with storing last map view state
• Fixed copying of the Object Alignment property when cloning a tileset
• Fixed the "Fix Tileset" button in the Template Editor
• macOS: Disable unified tool bar entirely (avoids rendering issues, #2667)