[Widgets] Add a new "Custom Header" widget

[wuyongfeng]

So this widget is to add a new "Custom Header" widget that would simply be a div.
Essentially it will then just act the same way the footer does.

[wuyongfeng]

Modify a bug

Geoff said:
"There's a significant bug here. I think it's the fault of this 'locator' selector - it will find all divs within the header, not just the one you're looking for.

Try dropping two buttons into the header, then put a Text widget between them, and change it to type 'h2' so they appear on three separate lines. Then drag a vertical button group into the header; try putting one in different places. Some of the widgets appear to be duplicated, I think because they're getting added to all divs, not just one."

The root cause is that the locator was "div", which will also match child nodes that have "div". We need to use "> div" to locate the immediate descendant.

[grgustaf]

Merged.