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">

relational_gui/resources.qrc

@@ -0,0 +1,5 @@
+<!DOCTYPE RCC><RCC version="1.0">
+<qresource>
+    <file alias="icon">resources/relational.png</file>
+</qresource>
+</RCC>