New feature: added support for type "label" to system.xml

[fballiano]

Since in #3577 there wasn't much consensus I tried a different approach.

Simply add this to your system.xml:

<testlabel>
    <label><![CDATA[Innermost dimensions, <strong>not including border</strong>, in pixels.]]></label>
    <frontend_type>info</frontend_type>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</testlabel>

and you'll get something like:

Closes #3577

[kiatng]

Tested.