Support for external resource loading via path strings for custom widgets in Designer

The request is about adding support for loading external resources for custom widgets in Designer.

Use case in more detail:

• There's a custom widget plugin that displays an image. The path to the image is provided as QString property e.g. ":/path/to/TestImage.png".
• The image is provided in resource file (attached to the form) by the user of that custom widget .

Currently with this setup when the custom widget is first created by Designer, the constructed QPixmap with the path provided as QString results in an invalid pixmap (isNull() == true).
If setting the image path is delayed e.g. with QTimer it will work and the image is shown properly.

Steps to reproduce:

• Build qtbug59132, run make install in the PluginTest
• Run the application: Note both images show
• Open PluginTestApp/mainwindow.ui in Designer. Note: below image does not show