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.
I'm new to Carat.  I haven't looked into using panels.  I moved forward with dialogs after seeing how the About dialog was done.  I'll look into the command palette or a new panel.