Provide way to programmatically manage kits

The `.pro.shared` file is very powerful for sharing project settings between machines. Unfortunately, it is not a complete solution to cloning a project across machines and operating-systems.

For a seamlessly cloning development environment, it should also be possible to programmatically set up a Build Kit. This is especially helpful for bare metal and custom Qt builds.

(The Maintenance Tool already does this, where it instructs Qt Creator to create or delete kits. However, this is not accessible outside the Maintenance Tool, and the Maintenance Tool only configures Qt Creator for detected Qt SDKs.)

A suggested way to implement this feature is to create a `.kit` file type. This would be double-clicked or otherwise loaded into Qt Creator.