DDSIDL: Cannot handle allowed values with blanks

vspec2ddsidl cannot handle string literals with included blanks, like below ( a blank after `WITH_`).

This is valid VSS (although not following VSS style guide), so vspec2ddsidl should preferably handle this, by ignoring/escaping so that the value becomes a legal DDSIDL value.

```
MySignal:
datatype: string[]
type: attribute
allowed: ['NO_BLANK', 'WITH_ BLANK', 'SMÖRGÅSTÅRTA']
description: string value with blank
```

Start by locating the vspec2ddsidl handling for allowed string values and reproduce the issue with the VSS example in this report. The work is done when values containing blanks, such as 'WITH_ BLANK', are converted into legal DDSIDL values without breaking the other shown values, including the non-ASCII text.

Written by the indexing model from the issue text.