Allow managing object template files

[bjorn]

Currently you can't manage the object templates in Tiled. You can only save new templates. The following actions should be added to the Templates view:

• Delete template (probably with a warning, especially if it's in use by any open map)
• Delete folder (probably only allow this when the folder is empty)
• Rename template
• Rename folder
• Move template (to another folder)
• Move folder

All these actions can currently be done outside of Tiled and the Templates view will update to reflect any changes. But that way template references will not be updated.