Changing variable name removes spaces in formalisation (and makes it hard to read)

[Langenfeld]

Reproduce:

1. Have formalisation with variable names such as A && B && C
2. Rename one of the variable to another name in the variables tab
3. Go back to formalisation, now it is A&&B&&C

Renaming of a single variable should not remove formatting of the string regarding spaces and line breaks.

[tkolzer]

Seems strongly related to #16