Add functionalities to fix invalid cells and clear all cells in GridMap

[Alex2782]

Fixes: #37124 (for 4.x)

This PR is partially based on #37136

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TODO: #37136 (comment)

... it should maybe be synced with how this is handled in the TileMap editor in 4.0 to be consistent (which is also able to display invalid cells in the viewport).

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Tried Clear All Cells, it works so far (also with Undo / Redo)