ENH: Add propertry for setting Qt Standard Icons in designer

[nstelter-slac]

allow qt standard icons to be set in designer by exposing a custom property that sets the icon using python inside the property's setter.

this should provide a set of default icons cross-platform, since they ship with qt.

[nstelter-slac]

to test:

-pydm examples/shell_command/shell_command.ui
-click on a shell command button
-copy any of the Qt Standard Icons, and set as the button's standardIcon property.
-should update immediately in designer, and in pydm if you re-run shell_command.ui

[klauer]

Neat! Could this be extended to support the fontawesome stuff built into PyDM as well - pydm.utilities.iconfont? https://github.com/slaclab/pydm/blob/master/pydm/utilities/iconfont.py#L1

[nstelter-slac]

can demo use of Qt's standard and Font Awesome icons with:

pydm examples/icons/icons.ui

[jbellister-slac]

Created a new display, worked exactly like I'd expect in both right hand window and basic settings editor in designer. Links for the valid icon names for both sets are good, example is helpful, and tests look good too. Thanks!