Use resource file

Use Qt resource files, compile it to Python to obtain a window that
has an icon!

Finally!

It is still using the Oxygen icon for Database, since I have nothing
better to use.

relational_gui/maingui.ui

@@ -20,6 +20,10 @@
   <property name="windowTitle">
    <string>Relational</string>
   </property>
+  <property name="windowIcon">
+   <iconset>
+    <normaloff>:/icon</normaloff>:/icon</iconset>
+  </property>
   <widget class="QWidget" name="centralwidget">
    <layout class="QVBoxLayout" name="verticalLayout">
     <property name="spacing">