Adds the Legend.ShowInvisibleSeries property to control whether invisible series should be shown on the legend

Fixes #1730, by providing a way to turn off the new behaviour. The default behaviour is true, such that the new behaviour is enabled by default.

When false, this property also disables the visibility toggling behaviour per #1723. With the various issues around events and the less-than-ideal situation of processing input upon hit-test, a more proper solution will be needed to #1723 in the future; however, this should suffice for 2.1, so that users can disable the new functionality if they wish.

Checklist

• I have included examples or tests
• I have updated the change log
• I am listed in the CONTRIBUTORS file
• I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• Add Legend.ShowInvisibleSeries property
• Don't show invisible series in a legend when ShowInvisibleSeries is false (fixes Invisible series are visible in the legend in version 2.1.0-Preview1 #1730)
• Don't toggle series visibility on hit-test when ShowInvisibleSeries is false (addresses Disable legend behavior in version 2.1.0 #1723 prior to a re-working of legend interaction)

Does this seem reasonable for the time being?
@oxyplot/admins