Add ability to search all files in the project

Adds a "Find in Project" option to the "Project" menu and a "search" function to the ProjectManager.  Regular expression and multiline queries are supported.  Clicking a result will open the file (if not open already) and move the editor's cursor to the appropriate line and column.  I have plans to make this a combination "Find / Replace in Project".  Let me know if you have any questions.
Hm, I'm also not sure that the best way to do this is by checking the activeElement. It seems like you're trying to put form elements into the dialog, which is fine, but we can figure out a better way to do this. More importantly, it's not really clear to me why we're using a dialog for this, instead of rolling it into the palette or adding a new panel the way that Sublime works.