Base the commercial license on the Royalty-free license adding clauses pertaining to the fees.

Add clarification that Application may not expose Slint APIs.

Updated the version from 1.1 to 1.2 
Renamed the header to "Slint Royalty-free Desktop, Mobile, and Web Applications License"
Added definition of "Mobile Application" and grant of right
Moved "Limitations" to 3rd section and "License Conditions - Attributions" to 2nd section
Added flexibility to choose between showing "MadeWithSlint" as a dialog/splash screen or on a public webpage
Moved the para on copyright notices to section under "Limitations"

The pre-commit hook is not enforced, don't scare contributors with it.

The testing section contains invalid link. The fact that it is tested by the CI is anyway mentioned in the testing.md doc linked from the document.
Fixes #4927

* Keep project related .md files triple licensed as the rest of the
  crate
* Make vtable MIT OR Apache 2.0 (as suggested by @ogoffart)

Instead, place the copyright and license right into the source.

To satisfy reuse, this also removes the unnecessary MIT.txt
symlinks.