Datagrid problems with keyboard

Nobody has claimed this yet.

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

Start with the demo app described in the issue and reproduce the keyboard behavior in MaterialDataGridComboBoxColumn. Inspect the datagrid column behavior, header tab order and focus indication, and MaterialDesignDataGridTextColumnPopupEditingStyle. Done means keyboard editing works, tab navigation and focus are usable, and the popup minimum width matches the cell width.

Written by the indexing model from the issue text.