Attribute values (in object diagrams) with newline and closing brace results in incorrect source

[hallvard]

If an attribute value contains a newline and a close brace, e.g. "Hello world }", the resulting diagram source code will be incorrect and does not parse. Problematic characters must be properly escaped or replaced/removed. It may also be helpful to shorten long strings with ... to avoid very large boxes in object diagrams.

[hallvard]

The fix replaces special characters by unicode arrows. Also does some truncation now, to avoid very long attribute values.