Use the property key instead of XML name in JSON paths

The property key is the exposed name for a property that is possibly
defined in a subproxy. Using this name makes it possible to set
defaults for these exposed properties, which wasn't possible before
for properties whose exposed name was different from the XML name.

Change-Id: If3e98511f9bce4023cc68b1f8d33bd8bb10413e2
Fix incorrect setting of XML name for exposed properties

Exposed properties from a subproxy were *almost* able to support
default values different from the subproxy's definition, but not
quite. The XML name was overwritten by the exposed name, which caused
updates from the Qt side to not propagate to the server. This patch is
a little bit of a hack that resets the XML name after the exposed
proxy XML attributes have been processed.

Change-Id: I3a99a1104b16436be3882170c982ce3b8a341798
Moved some custom property values to XML

Now that default values can be specified for exposed properties of
subproxies, moved custom default for color legend font sizes to
XML.

Change-Id: Iada427a2269c52af119281e562648637ae0e43c7
Improve pqProxyWidgetDialog sizing

Change-Id: I19b196ccee8084a8ce44a3d6026bf34b7d09dea3
Marked some properties with no_custom_default attribute

This prevents these setting values from being saved to the
user settings file.

Change-Id: I5023cfd99ccfb6a4c49d4333b09bcc551a29351a
Add buttons to pqProxyWidgetDialog

Added a button to save currently applied settings as property defaults
and a button to restore the application defaults.

Removed QDialogButtonBox and replaced with simple QPushButtons.

Updated tests that involve the pqProxyWidgetDialog.

Change-Id: Iedc7684af5741b0854470ace3dcb459e5d523999
Merge topic '14762_save_color_legend_settings' into next

4b05cb56 Add buttons to pqProxyWidgetDialog
111b9a80 Marked some properties with no_custom_default attribute
30c1500f Improve pqProxyWidgetDialog sizing
b65c7f2b Moved some custom property values to XML
1838197c Fix incorrect setting of XML name for exposed properties
6263ae4a Use the property key instead of XML name in JSON paths