Use TextBox instead of TextBlock for displaying read only property

[Nfactor26]

Issue Description

1. TextBlockEditor : TypeEditor is used in PropertyGrid for displaying readonly properties.
   However, TextBlock control in WPF doesn't allow to override it's Template using a style.
2. Margin is set to (5,0,0,0) from inside SetControlProperties() of TextBlockEditor which takes precedence over any margin applied from style.

Changes

1. Created a new ReadOnlyTextBoxEditor which uses TextBox to display readonly property
2. The default style overrides control template of textbox such that it uses a TextBlock with TextTrimming="CharacterEllipsis" and Margin="5,0,0,0" similar to what TextBlockEditor was using.
3. Updated TextBlockEditor initialization at different places to intialize ReadOnlyTextBoxEditor instead.

**Impact on existing users **

1. Those using default styles won't notice any difference in UI since TextBlock will be used to display readonly property
2. Those who have customized style for TextBlockEditor will have to create a new style for ReadOnlyTextBoxEditor instead.

** Benefit **
Users will now be able to provide a custom control template in styles for displaying their readonly property which was not possible before with a TextBlock.

[XceedBoucherS]

Hi,
You are right, TextBlockEditor.SetControlProperties() modify the Margin and TextTrimming of the editor so a style can't override those values. But if we remove those 2 properties from TextBlockEditor.SetControlProperties() and set them in the style targeting PropertyGridEditorTextBlock (from Aero2.NormalColor.xaml), it fixes the issue.

A user then simply needs to do this in is xaml in order to modify the style:
<Window.Resources> <Style TargetType="{x:Type xctk:PropertyGridEditorTextBlock}"> </Style> </Window.Resources>

This will be fixed in v3.9.
Thank you.