SplitView - Add view provider mechanics

[pthiess]

Part of the Split View feature in Brackets 1.0.

Dependencies

• SplitView - Implement MainViewManager code for 1x2 editors

Tasks

• implement view factory and MainViewManager.addView()
• Register view provider
• Implement required interfaces (ImageView, Editor)
• Create image view provider
• Refactor pane view lists to support things that are not file objects
• Implement all file based commands to work on images
• Implement Command.FILE_ADD_TO_WORKING_SET to resolve to a doc
• Add checks in DocumentManager working set APIs to filter for Documents
• Implement ViewStateManager
• Rework EditorManager.canOpenFile
• Kill addCustomViewer
• Unit tests
• Implement PR Jeff/couzteau fix 6356 stale image update #7952
• Rework MainViewManager.open to be more efficient
• Add support for arbitrary views

[pthiess]

PR #8753