Impute: allow user-defined value for "no data"

**What's your use case?**
In Orange, cells with missing data are shown as "?" in Data Table. These are also the cells that are affected by impute. But in some standards, missing data is represented by some fixed value. For instance, in files produced by Garmin tracking devices, a missing heart rate is shown as "255" (apparently this is a heart rate never measured in real humans, also the max. value fitting into 8 bits).

**What's your proposed solution?**
Allow user specification of the value that corresponds to "no data"

**Are there any alternative solutions?**
Convert the "no data" to "?" using Formula, e.g., `float (nan) if heart_rate == 255 else heart_rate` - however this is not a very straightforward solution.

Start by locating the Impute component and its handling of missing values, then compare it with the Formula workaround described in the issue. Define how a user-specified sentinel such as 255 should be recognized and verify that the resulting cells behave like missing data in Data Table and imputation; the issue is stale and has had no recent response.

Written by the indexing model from the issue text.