Use python's webbrowser instead of Qt.openUrlExternally for opening urls

Qt.openUrlExternally crashes on Windows, but python webbrowser works. So
use that instead.

Fixes Asana issue 33694049548892

cura/CuraActions.py

@@ -0,0 +1,15 @@
+from PyQt5.QtCore import QObject, pyqtSignal, pyqtSlot, pyqtProperty
+
+import webbrowser
+
+class CuraActions(QObject):
+    def __init__(self, parent = None):
+        super().__init__(parent)
+
+    @pyqtSlot()
+    def openDocumentation(self):
+        webbrowser.open("http://ultimaker.com/en/support/software")
+
+    @pyqtSlot()
+    def openBugReportPage(self):
+        webbrowser.open("http://github.com/Ultimaker/Cura/issues")