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

从 issue 中描述的演示应用开始，复现 MaterialDataGridComboBoxColumn 中的键盘行为。检查 datagrid 列的行为、标题的 Tab 顺序和焦点指示，以及 MaterialDesignDataGridTextColumnPopupEditingStyle。当键盘编辑正常工作、Tab 导航和焦点可用，并且 popup 的最小宽度与单元格宽度一致时，即视为完成。

由索引模型根据 Issue 内容生成。