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

Comienza con la aplicación de demostración descrita en la issue y reproduce el comportamiento del teclado en MaterialDataGridComboBoxColumn. Inspecciona el comportamiento de la columna del datagrid, el orden de tabulación y la indicación del foco, así como MaterialDesignDataGridTextColumnPopupEditingStyle. Se considera completado cuando la edición mediante el teclado funciona, la navegación con tabulador y el foco son utilizables, y el ancho mínimo del popup coincide con el ancho de la celda.

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