fix(gridMenu): GridMenu with multiple grids & event leak

[ghiscoding]

• GridMenu was not working correctly with multiple grids
• GridMenu was not showing correctly in a Modal Window because it's z-index was too low
• Also found event leak, GridMenu should be destroyed on grid destroyed

Tested

• Tested with 2 grids in the same page
  • Commands clicked on 1st grid Grid Menu should not affect 2nd grid
• Destroying 2nd grid should destroy it's GridMenu as well & it's events attached without affecting the GridMenu of 1st grid
  • After 2nd grid is destroyed, we test that 1st GridMenu still work as expected