Appearance: add action to reset custom background color

[lancewillett]

In Appearance > Background form there is no way to clear the color hex value other than knowing that you can remove the hex value itself by hand.

Patch adds a "Clear" link that removes the color value using the pickColor method by passing it an empty string.

[nathanrice]

+1

[lancewillett]

@nacin Would it be OK to get this into 3.2 now?

[lancewillett]

Refreshed patch for trunk as of r17707

[nacin]

We can probably pull the CSS into the header, rather than inline.

In the future, no need to do compressed styles/scripts. We handle those on commit.

[lancewillett]

Replying to nacin:

In the future, no need to do compressed styles/scripts. We handle those on commit.

Noted, thanks.

[ryan]

Redacted comment. Wrong ticket.

[nacin]

In [17886]:

Add clear to custom background color field. props lancewillett for initial patch. fixes #15755.