Datagrid problems with keyboard

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the datagrid `MaterialDataGridComboBoxColumn` can't be used with keyboard

when i get to the column with my keyboard by pressing "Tab" pressing any key to open the "drop down list" including space doesn't do anything and i have to use the mouse to do so even if i forced the editing cell mode for it

can't set the `MaterialDataGridComboBoxColumn` to editable

pressing tab to go throw app fields work fine until i reach a datagrid instead of going from the last field to the first cell in the first row if there is any it will go throw the columns headers first then the first row.
if you need this for a checkbox to select all or something just add `IsTabStop` with false as the default also there is no visible indication where it currently focus on with the headers because the dots box doesn't show on them

the `MaterialDesignDataGridTextColumnPopupEditingStyle` should popup with minwidth as wide as the cell width.

demo https://youtu.be/d21YR-9FKk4

you can test all of this in the demo app, thanks

Inizia con l’app demo descritta nell’issue e riproduci il comportamento della tastiera in MaterialDataGridComboBoxColumn. Ispeziona il comportamento della colonna del datagrid, l’ordine di tabulazione e l’indicazione del focus, nonché MaterialDesignDataGridTextColumnPopupEditingStyle. Il lavoro è completato quando la modifica da tastiera funziona, la navigazione con Tab e il focus sono utilizzabili e la larghezza minima del popup corrisponde alla larghezza della cella.

Scritto dal modello di indicizzazione a partire dal testo della issue.