Make period a nullable property of reactive timer and clarify documentation

[glopesdev]

Ignoring the value of TimeSpan.Zero for the Period property is an unfortunate historical legacy. The property should really expose a nullable type when deciding between which Observable.Timer overload to call.

This PR changes the type signature of Period to be a nullable timespan, but retains the same behavior by requiring Period to be larger than zero for periodic timers. We also introduce a slight modification to its serialization behavior to ensure that a value of zero is always serialized as null.

This will start the migration away from using TimeSpan.Zero as a valid value, and might give us some room to consider reinstating the zero period as a high frequency periodic timer for 3.0.

We could also introduce an automatic conversion at the level of the editor, but this might break embedded include workflows which happen to be using the old timer semantics, since these are not converted automatically by the editor until they are ungrouped and resaved.

The embedded description string was also fixed for the shaders timer.

Fixes #1947

[PathogenDavid]

Just a nitpick over wording, could go either way