Improvements to JVersion

[nathanwbrei]

• Feature: constexpr JVersion arithmetic
• Feature: JPluginLoader now searches for plugins in CMAKE_INSTALL_PREFIX
• Feature: Version description now includes install prefix and optional dependencies
• Bugfix: stderr suppression while generating JVersion.h
• Refactoring: Move version and splash printers out of JMain and into JVersion
• Refactoring: Call version and splash printers from JApplication::Initialize instead of JMain

[nathanwbrei]

This completes most of the issues mentioned in #268.

• I decided against adding a constexpr JVersion(int,int,int) constructor with overloaded comparison operators because JVersion is much richer than a 3-tuple of ints, and because it would have exactly the same power as doing JVersion::GetVersionNumber() < 2'003'001 but with a whole lot more complexity.

• Checking the version number when loading plugins is really more a JPluginLoader problem, so I'll add it to the list of JPluginLoader problems and tackle them together. The plugin loader is tricky to modify because it has lots of finicky details and doesn't lend itself well to unit testing.