Replace sexpresso library with own implementation

[ubruhin]

Replacing sexpresso::parse() with our own parser in the SExpression class. See #783 for the motivation behind this change.

Because we also used sexpresso::escape() to escape characters in strings to be serialized, I also had to replace this by our own implementation. Then I saw that sexpresso::escape() even replaced ? by \?, ' by \' and the audible bell by \a (wtf?), which doesn't make sense to me 😉 So I removed these replacements, but only for LibrePCB 0.2.x to keep the 0.1.x file format as-is.

Positive side effect of the own implementation: It seems to be much faster than sexpresso::parse() 😃 Not an accurate measurement, but on my computer the background library scan now takes ~25% less time than before (tested only in debug mode).

I just hope there are no critical bugs 🙈 I added some unit tests and tested manually by opening/saving whole projects, at least so far it seems to work properly...

• Fixes Get rid of "sexpresso" dependency #783.
• Useful for Unbundling libraries lead to messy project files and doesn't work reliable #792 since sexpresso no longer needs to be unbundled

[dbrgn]

The parser would be a good candidate for fuzzing, e.g. using afl++. Can be done at a later point in time though.