Updates from https://github.com/dirkf/stylem

[dirkf]

I forked the project to apply some fixes for SeaMonkey 2.5.13.

Now that most of those fixes have been separately merged here thanks to @UCyborg, I thought I'd offer the remaining changes from that fork.

This PR adds:

• additional changes from an old Stylish PR (see also here), other than those that are replicated by UCyborg's recently merged fixes;
• a safety fix to avoid console warnings and some simplification in src/stylem/src/content/overlay.js;
• a couple of documentation updates.

Not that I have a Firefox 57 at hand to check, but I believe that with these changes in places you could also apply this:

--- old/src/install.rdf
+++ new/src/install.rdf
@@ -78,7 +78,7 @@
       <Description>
         <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
         <em:minVersion>52.0</em:minVersion>
-        <em:maxVersion>52.*</em:maxVersion>
+        <em:maxVersion>57.*</em:maxVersion>
       </Description>
     </em:targetApplication>
     <!--Thunderbird-alikes-->

[Lootyhoof]

LGTM (and importantly, still works as expected in Pale Moon). I'll push out another release soon with these fixes included.

Thanks!