Fix broken TOC of PDFs if section includes an image

If someone adds an image to a section it will be added to the sidebar
TOC of a PDF, too. A PDF viewer will show the "width and height"
data instead of an image. So we need to filter out images for this with
the optional "short title" of latex sections.

tests/roots/test-image-in-section/conf.py

@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+
+master_doc = 'index'
+
+rst_epilog = '''
+.. |picture| image:: pic.png
+    :width: 15pt
+    :height: 15pt
+    :alt: alternative_text
+'''
+

tests/roots/test-image-in-section/index.rst

@@ -0,0 +1,18 @@
+test-image-in-section
+=====================
+this is dummy content
+
+
+|picture| Test section
+----------------------
+blah blah blah
+
+
+Another section
+---------------
+another blah
+
+
+Other [blah] |picture| section
+------------------------------
+other blah