I've made the following changes:
- make `is_active` the main way of keeping track of tween
processing/activity, meaning that `is_active` will now return
`false` if all tween actions have finished or if it isn't started
or if it was stopped via `set_active(false)` or any other mode
- removed is_stopped because is redundand now
The above meant that we don't have to keep track of yet another variable
`available` since everything is based on `*processing_internal` so I
removed it, likewise it's own local `processing` variable was removed,
as well as the "double" `_set_process` which it feels more like a hack.
What wasn't changed:
- `tell()` still returns max value (i.e. `== get_runtime()` when all
tweens `finish`)
*More testing is needed*. So far I've tested repeat on/off, delay,
`is_active()` working corretly, `set_active(true), set_active(false)`,
but probably more tests are necessary, all the resets, stops, resume
etc.
5c914e2d5b