fix some small stuff, add icons to main UI, add shortcuts

src/ui/main_ui.py

@@ -100,6 +100,15 @@ class MainUI(QtWidgets.QMainWindow, Ui_MainWindow):
             log.info("Backup enabled")
         else:
             log.warning("Backup disabled")
+
+        # set Action Icons
+        Icon("settings", self.actionEinstellungen)
+        Icon("user", self.actionNutzer)
+        Icon("quit", self.actionBeenden)
+        Icon("report", self.actionBericht_erstellen)
+        Icon("history", self.actionAusleihhistorie)
+        Icon("help", self.actionDokumentation_ffnen)
+        Icon("support", self.actionProblem_melden)
         self.show()
 
     def shutdown(self, *args):