design violation: user_mode_view should be separated from models via a controller or must have indirect model imports.

[rajdeepadak]

All operator logic is placed within models. A model (containing the logic) can be called by a controller when a user interacts with a view item. Therefore, header menu operators and panel views should be separated from user mode view.

[rajdeepadak]

Implemented a callback controller to blockmesh related layout categories with issue #6.