Clean up plugin.xml "definition" extensions

[AaronGreenhouse]

The plugin.xml extension point org.eclipse.core.expressions.definitions allows expressions to be named for later use in the reference tag. Many of these are declared throughout the OSATE plugins. We need to determine which ones are general enough to be reused by 3rd parties and document them.

[AaronGreenhouse]

Did a bunch of this with Issue #841. Seem appropriate to do it there.

[AaronGreenhouse]

Created https://github.com/osate/osate2/wiki/Standard-Expression-Definitions

Not closing this yet as it is a work in progress.

[AaronGreenhouse]

Going to close this for now. Did the cleanup on the branch for issue #841 as said already, and created a wiki page to document it.