XML mode won't unindent if autoCloseTags is disabled

[marcelgerber]

(taken from #8980 (comment). I created a new issue as XML/HTML is definitely more of a web language than Verilog)

1. Go to your brackets.json and set closeTags: false
2. Create a new file test.xml
3. Type <name>
4. On the next line, type </name>

Then try steps 3-4 in http://codemirror.net/mode/xml/

Result:

<name>
    </name>

Expected:

<name>
</name>

That's because in our Editor._checkElectricChars function, we assume languages will only ever use the electric chars ([{, but CM has a per-mode electric chars option (currently only used by XML and Verilog).

[JeffryBooher]

Closing as duplicate of #8980