Replace BlockEditorView.show(blockDef)

[AnmAtAnm]

BlockEditorView.show(blockDef) duplicates code/functionality in .showBlock(blockDef), and does edit "tab" manipulation without involving the AppController.

Replace the function with a call in AppController that calls (directly or indirectly) BlockEditorView.showBlock(blockDef).

[AnmAtAnm]

The different code paths through BlockEditorView.show(blockDef) and .showBlock(blockDef) is probably partially responsible for #297.