MaterialDesignDataGridTextColumnPopupEditingStyle: Mouse and focus issues

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When you double click on a cell which has `EditingElementStyle="{StaticResource MaterialDesignDataGridTextColumnPopupEditingStyle}"` defined, the popup appears but there are a few issues:

1) You can't move the cursor using the mouse and can't select text with the mouse (although right clicking brings up the context menu correctly).
2) If the cell editing popup is open and you change focus to another app, the popup remains on top of the other application.
3) (Minor/opinion) The cursor's position is at the beginning of the TextBox when the popup editor opens (I think this would typically be at the end?)

To reproduce:
- Open the WPF demo
- Selecting the "Grids" page using the menu
- Double click on a cell in the _Name_ or _Description_ columns to start editing
- Try using the left mouse button to move the cursor or select text inside the popup
- Switch to another application while the cell editor is open

Here's a screenshot of the popup over another window.

Start with the WPF demo's Grids page and the MaterialDesignDataGridTextColumnPopupEditingStyle used by the Name and Description columns. Reproduce the popup behavior with a double-click, then trace the popup editor's mouse selection, application-focus, and initial cursor-position handling. Done means mouse editing works, the popup does not stay above another application, and the cursor behavior is addressed.

Written by the indexing model from the issue text.