Don't return an Option, just return 0 when the timer is not started.
As discussed in the API review, the rational is that the interval is
just like a field in a struct and when the struct is default
constructed, it is initialized to 0
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"
We have a nice FFI-compatible wrapper areound rust timers in C++, so
allow to use that in other types exported to C++.
This also makes sure the rust and C++ side agree on the size of the
type, with the nice side effect that the rust Timer now uses half as
much space now as it did before.
