Fix TreeDataGridTextCell changing TextCell.Value

[erri120]

TreeDataGridTextCell listens to property changes of ITextCell.Value to update its own Value property. The setter of TreeDataGridTextCell.Value updates ITextCell.Text to the string representation of the new value.

This is done because the cell can be edited, and
TreeDataGridTextCell.Value is bound to TextBox.Text two-way when the user wants to edit the cell.

However, TextCell<T>.Text which the setter of TreeDataGridTextCell.Value will always set, doesn't respect the read-only status or the editing status of the cell:

1. TextCell<T>.Value updates.
2. TreeDataGridTextCell reacts and sets TreeDataGridTextCell.Value
   to the string representation of the new value.
3. The setter of TreeDataGridTextCell.Value will set TextCell<T>.Text
   to the string representation.
4. The setter of TextCell<T>.Text will try to convert the text
   representation of the new value as the new value...

If it weren't for the fact that RaiseAndSetIfChanged does what it's supposed to do, which is only raise if changed, this would be a loop that goes on forever.

Users will get an exception if T of TextCell<T> fails to convert: Convert.ChangeType(value, typeof(T)). This obviously isn't an issue when T is string but it does have issues for base types like numbers (localization issues), and custom types will almost always result in an exception.

[erri120]

The Modified_Text_Is_Written_To_Binding test is failing because it does

target.Text = "new";

without using BeginEdit. I'm fine with removing the !_isEditing check from the TextCell<T>.Text setter to prevent API breakage.