Editor: cleanup test_CustomPlugin.html example

[bill]

Fix and simplify test_CustomPlugin.html example code.

• test_CustomPlugin.html code is getting errors (see IE6, it prints "syntax error" in the console, twice)
• the dialog header and horizontal bar don't show up correctly on IE; for purposes of example, can just remove them (dropDown.title + "<hr>")
• i18n stuff is not needed for the example (hardcode strings instead)
• code for URL regex is not needed
• code for <img> tags is not needed (anywhere "img" appears in the code)
• further simplifications based on above changes (removing variables, etc.)

[peller]

I'd rather do a different plugin than the image plugin anyway, since that's just duplicating code in the project. Maybe view-source?

[peller]

(In [13685]) Replace custom plugin example with something simpler and unique -- toggle html source. Refs #6224

[peller]

(In [13686]) Set custom label. Refs #6224