Fixed display issues with oversized window

If the window defined in src/mainwindow.ui exceeds the size of the
display, the program behaves weirdly. Growing the window size even further
leads to crashes. Reducing the window size to 1x1px seems to work fine
and should be smaller than any display size.

src/mainwindow.ui

@@ -6,8 +6,8 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>1500</width>
-    <height>945</height>
+    <width>1</width>
+    <height>1</height>
    </rect>
   </property>
   <property name="sizePolicy">