Change the Properties Panel button slots to be public

This changes the apply()/reset()/deleteProxy()/showHelp() slots
in the pqPropertiesPanel class to be public instead of private.
This allows for the application code to programmatically click
the buttons on the properties panel.

Change-Id: I42de8f35f14c0b8ac26c7bddfa730e222ff7751f
Merge topic 'properties-panel-public-apply' into next

082da13 Change the Properties Panel button slots to be public