Support for \' \` \^ \~ \= \u \. \" \r \H \v text-mode accents (#802)

* Support for \' \` \^ \~ \= \u \. \" \r \H \v text-mode accents

* bug fix

* Possible Safari fix

* Forbid text accents in math mode

* Switch to noncombining characters for most accents. Revert Safari change.

* Add tests

* Found non-combining diaresis accent too

test/screenshotter/ss_data.yaml

@@ -12,6 +12,11 @@
 # on the next line. See http://www.yaml.org/ for syntax details.
 
 Accents: \vec{A}\vec{x}\vec x^2\vec{x}_2^2\vec{A}^2\vec{xA}^2
+AccentsText: |
+    \begin{array}{l}
+    \text{\H{e}}\\
+    \text{\H{X}}\\
+    \end{array}
 Aligned: |
     \begin{aligned}
         a &= 1 & b &= 2 \\