Changing "text" setting makes Link lose track of which variables need saving

I had already a text variable `History` that was saved in a file `History.apla`.

Recently I added `text:plain` to the Link configuration file.

In such a case the contents of the file `History.apla` is correctly established on a variable `History` in the APL workspace. But because of the new setting, when one changes the variable in the editor, the change is **_not_** written back to file by Link.

The reason for this is that Link compiles the filename as `History.vec.apla`, and then checks whether that file is already on disk. It's not, and because of that Link does not do anything.

That's not what should happen. As a user of Link, of course I expect Link to process this accordingly:

* Save the changes in `History.vec.apla`
* Delete the file `History.apla`