Add placeholder header pattern

This removes a warning from our log that this image couldn't be found. Instead it now draws a completely transparent image there.
The warning that is no longer in the log now looked like this:
2020-03-20 13:52:27,980 - WARNING - [MainThread] UM.Qt.Bindings.Theme.getImage [133]: No image header_pattern defined in Theme

Done during Turbo Testing and Tooling.

resources/themes/cura-light/images/header_pattern.svg

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<svg width="10" height="10" xmlns="http://www.w3.org/2000/svg" version="1.1">
+    <!-- This file intentionally left empty. -->
+    <!-- A theme might fill this in if it wants a pattern in the header banner. -->
+</svg>