SplitView - Implement MainViewManager code for 1x2 editors

[pthiess]

Part of the Split View feature in Brackets 1.0.

Dependencies

• SplitView -Add view provider mechanics
• SplitView - Refactoring

Tasks

• Move "editorAreaResize" event to MainViewManager
• Implement New Editor APIs
• Implement editor create
• Scaffolding (Temporary) UI
• Implement Pane Management (maps the working set to the pane)
• Migrate Editor Management from EditorManager to Pane
• Deprecate Editor Manager APIs
• Update titlebar on document switch
• Move "currentDocument" management and EditorManager "currentlyViewedPath" to pane as "currentView"
• Implement WorkingSetView Creation
• Implement working set view context and gear menus
• Implement "merge" splitviews (the unsplit)
• update close others extension
• Implement serialization code
• Rejigger working set gear menu and preferences
• Implement View Layout APIs
• Unit tests

[pthiess]

PR #8913