(fwPreferences): add getValue helper

What does this MR do?

The method getPreferenceKey(...) returns associated value saved in the preferences (if delimited with the character %) or simply returns the variable. This helper is defined (copied/pasted) 12 times in Sight. To avoid this, we have centralized all these versions in fwPreferences with the new method getValue(const std::string& var, const char delimiter = '%').

How to test it?

configure/build sight and run the unit-test:

> bin/fwpreferencestest

Data

no required data

Additional tests to run

• Build on Windows
• Build on Linux
• Build on Macos

Associated Issues/Merge Requests

• Issues
  • Closes #304 (closed)