Support default parameters for PCF control properties

[ashlega]

Hi,

  this one is based on the community thread here: https://powerusers.microsoft.com/t5/PowerApps-Component-Framework/Default-values-for-quot-input-quot...

  It might be useful to have the ability to define default values for the PCF control "input" properties AND to show those defaults in the form designer when adding the control. 

  I am guessing default values could either be added through the manifest file:

  <property name="test" display-name-key="test" description-key="test" of-type="SingleLine.Text" usage="input" required="false" default="default value" />

  Or from the code...

  And those default values should also show up in the form designer whenever somebody is setting up a custom control for an attribute on the form (so it would be obvious what the defaults are)