add discover_gui (without introducing ui dependencies !)

pyblish/gui.py

@@ -0,0 +1,37 @@
+import pyblish.api
+
+
+def show(parent=None):
+    """Try showing the most desirable GUI
+    This function cycles through the currently registered
+    graphical user interfaces, if any, and presents it to
+    the user.
+
+    parent (QWidget): the Parent widget to pass to GUI, preferably parented under QApplication
+    to avoid any dependencies on Qt, pyside2, or your favorite QT wrapper:
+    if you don't have a QApplication instance, instantiate one yourself
+
+    return a QWidget instance
+    """
+    gui_show = _discover_gui()
+
+    if gui_show is None:
+        raise RuntimeError("No GUI found")
+    else:
+        return gui_show(parent)
+
+
+def _discover_gui():
+    """Return the most desirable of the currently registered GUIs"""
+
+    # Prefer last registered
+    guis = reversed(pyblish.api.registered_guis())
+
+    for gui in guis:
+        try:
+            gui_show = __import__(gui).show
+        except (ImportError, AttributeError) as e:
+            print("Failed to import GUI: %s" % gui, e)
+            continue
+        else:
+            return gui_show