Make properties fully thread-safe

Title:	Make properties fully thread-safe [[File: |thumbnail|]]
Potential mentors:	Julian Smith
Intro:	(short intro)
Interested Parties:	(feel free to add yourself)
Status:	Post-hackathon write-up (2021-11-7) Summary: Changed properties API to make getStringValue() return a std::string by value. Modified code in flightgear and simgear to use new API. 3,000 line diff. Appears to run fine except for two issues: Approx 10% slow-down (found using the new Performance testing by replaying recordings system developed in the same Hackathon). The Aircraft menu is corrupted - all items are displayed with the same text "Translations/defualt/menu.xml". Next steps: Fix the cause of the menu bug. Write a stress test - lots of threads all reading/writing properties, adding/removing listeners etc. Investigate the slow-down: Hopefully we will be able to change calling code to avoid the overhead. The point was made in the final Hackathon meeting that a 10% slowdown might not matter too much if thread-safe properties allowed us to make better use of threads throughout Flightgear.
Summary:	Change property system to return std::string by value instead of char*.
Background:	The properties system was made mostly thread-safe in early 2021 using read/write locks (see SimGear commit f1dd7901cbb9ed5fb342ae1ec2ac982b915fb9f0). However some methods such as SGPropertyNode::getStringValue() still return a char* pointer into the property's internal data, which is not thread-safe.
Details:	Returning std::string by value might affect performance, so we'll need to profile before/after. It's likely that we can fix performance problems by modifying calling code, e.g. caching string values outside loops etc.
Ideas:
Required skills:	C++
Learning Opportunities:
Notes:

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