favicon: Add a script to generate numbered favicons

(imported from commit a80515c97a198eb2effaadcc578eeb4bf4c6b327)

assets/favicon/.gitignore

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+/tmp.svg

assets/favicon/generate

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+#!/usr/bin/env python
+import xml.etree.ElementTree as ET
+import subprocess
+
+# Generates the favicon images containing unread message counts.
+
+# Open the SVG and find the number text elements using XPath
+tree  = ET.parse('orig.svg')
+elems = [tree.getroot().findall(
+    ".//*[@id='%s']/{http://www.w3.org/2000/svg}tspan" % (name,))[0]
+     for name in ('number_back', 'number_front')]
+
+for i in xrange(1,100):
+    # Prepare a modified SVG
+    s = '%2d' % (i,)
+    for e in elems:
+        e.text = s
+    with open('tmp.svg', 'w') as out:
+        tree.write(out)
+
+    # Convert to PNG
+    subprocess.check_call(['inkscape', '--without-gui', '--export-area-page',
+        '--export-png=../../zephyr/static/images/favicon/favicon-%d.png' % (i,),
+        'tmp.svg'])