Internal (Refactors): Minor refactoring

* BNFE_CONSTANTS: Untitled string constant
* BNF_DOCUMENT: reset feature and renames
* Removed BNF_GRID_* and BNF_SPECIFICATIONS --> singular name version
* ROOT become SPECIFICATION
* BNF_VIEW: enable_visible comes back
* BNF_MAIN_WINDOW: Some preliminary refactor of loading document
*