DevTools: Make SourcesNavigator a standalone component in SourcesPanel.

Moved file creation/renaming logic to navigator.
Made navigator listen for workspace changes itself.
Simplified file selection in navigator handler in sources panel.
Drive-by: Revealing uiSourceCode in navigator is not done automatically anymore, it is added as a context menu action instead now.

R=pfeldman, lushnikov

Review URL: https://codereview.chromium.org/196573009

git-svn-id: svn://svn.chromium.org/blink/trunk@169129 bbb929c8-8fbe-4397-9dbb-9b2b20218538