Missing strings for search

[k88hudson]

• "Change search engine"
• "Search with ${engine}"

I think these were passed as html elements before. We should either pass these strings through the search api, or pull down the locale file for search

[sarracini]

list of search strings that we need to pull down:

• searchPlaceholder = "Search"
• searchHeader = "%S Search"
• searchForSomethingWith = "Search for <span class='contentSearchSearchWithHeaderSearchText'></span> with:"
• searchWithHeader = "Search with:"
• searchSettings = "Change Search Settings"

Since we're not doing the api stuff anymore, these can be received through the message passing that's already set up for NewTab:ContentSearchService in CommActions.js (similar to suggestions). Alternatively we can just add them to the localization files, and just grab them from there. Thoughts @k88hudson?

[sarracini]

Actually thinking about this now, we have to get them using a message, because about:home uses the same search stuff. So if the search strings need to be changed one day, they need to change in one place only. They do get cached however on the WebIDL side, so unnecessary messages between MozSearchContent.js and ContentSearch.jsm don't get sent. I guess when we remove webidl stuff this won't be relevant though and they'll be used getting a message anyways.