"Keep effects running without input" togglable per-effect

### Enhancement Summary
Enable specific effects to have infinite decay (when "keep effects running without input" off).
Alternatively give user-control to disable constant processing on individual effects so decay and gate can be modified on a per-effect basis.
In this event, any toggle in the settings could simply signify the default- if necessary.

Additionally, on projects where decay and gate are connected to automation track, LMMS could usefully temporarily disable "Keep effects running without input" during playback and running of the project.

#### Justification
We cannot remove the ability for controlled gate length, and keep effects running by default. I have seen projects that use distortion plugins that add a base level of noise, created with the expectation that "keep effects running" is off, and automated decay so the
If using long-running reverb/echo alongside a distortion that adds noise, we need to be able to control for both. In the event eight seconds of decay isn't enough (which it may not be; additionally it is ugly to pull the decay up each effect up unless it is noisy) the ability to control per-effect would be useful.

Related: #4284
It has been suggested to remove the toggle altogether. Due to above mentioned reasons this is not possible.

Start by reading the existing “Keep effects running without input” behavior described here and the related issue #4284. Define how per-effect decay and gate controls should interact with the global default and with automation during playback; the work is done when those cases have an agreed, testable design.

Written by the indexing model from the issue text.