[Mac] Expose API to hide/show property button

[BretJohnson]

In general, for MonoDevelop in 8.4 they never want to show the property button as it doesn't do anything (Reset property is always disabled, in 8.4).

We should probably have a setting named ShowPropertyButton or similar to hide it, similar to the ShowHeader flag added earlier for #663.

That can remove the unneeded button from the Unity preferences here:

And IDE properties here:

See discussion at https://xamarinhq.slack.com/archives/C0YC6U677/p1573615723262800

The ability to reset properties to default might come in a later release of VSMac, for IDE settings, so we can show this UI later if needed, but it isn't there now.

Having this be a global panel setting, rather than per property, allows the entire column to be hidden.