Datagrid problems with keyboard

まだ誰も着手していません。

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

issue に記載されているデモアプリから始め、MaterialDataGridComboBoxColumn のキーボード動作を再現します。datagrid の列の動作、ヘッダーのタブ順序とフォーカス表示、および MaterialDesignDataGridTextColumnPopupEditingStyle を調査します。キーボードによる編集が機能し、タブナビゲーションとフォーカスが使いやすく、ポップアップの最小幅がセル幅と一致すれば完了です。

索引モデルが issue の本文から書いたものです。