Multiple gridmaps in same scene cause issues with mesh selection and filtering

### Tested versions

3.5

### System information

Windows 10

### Issue description

If using multiple gridmaps in the same scene and switching between them, the editor will try to pick the MeshLibrary item from the previously selected gridmap's meshlibrary and choose it in the newly selected gridmap (instead of the one previously chosen in that gridmap) and if using the filter meshitem and less than two are chosen, it is impossible to pick the only chosen meshitem.

The chosen item should be separated between each gridmap, so switching between them keeps the item chosen last independently.

Similar to https://github.com/godotengine/godot/issues/64126 but without the error, it's still very inconvenient though.

### Steps to reproduce

- Add two gridmaps to a scene
- Have them both use separate MeshLibraries of different sizes
- Pick an item from one gridmap meshlibrary for painting with and switch to the other gridmap
- Godot will attempt to select the previously chosen item from the other gridmap in the newly chosen gridmap, and if filtering a meshitem library that results are less than two, you won't be able to choose the single selected meshitem.

### Minimal reproduction project (MRP)

N/A

Reproduce the issue with two GridMaps using separate MeshLibraries, then switch between them while selecting and filtering mesh items. Trace where the GridMap editor stores the selected item and filtered results. Done means each GridMap independently restores its last selected item and a single filtered result remains selectable.

Written by the indexing model from the issue text.