[devtools] chrome.devtools.panels.sources.createSidebarPane

Add functions to chrome.devtools extension API:
  chrome.devtools.panels.sources.createSidebarPane()
  chrome.devtools.panels.sources.onSelectionChanged.
    add/removeListener(function(selectionInfo) {})
       selectionInfo: url start/endLine start/endColumn

Largely parallel to panels.elements. Main impl difference:
  the selection event originates within components of the panel,
  requiring addition of tabbedEditorContainer accessor to SourcesPanel.

Extend LayoutTests from panels.elements to cover panels.sources.

BUG=295888

Review URL: https://chromiumcodereview.appspot.com/23904024

git-svn-id: svn://svn.chromium.org/blink/trunk@158340 bbb929c8-8fbe-4397-9dbb-9b2b20218538