Remove margin around TextEditor

[mgarstenauer]

This is a cosmetic PR that removes the 30px margin around the TextEditor in the demo.

Before:

After:

(Looking at the git history, the Margin has been there for the last 7+ years. I don't think that the margin serves any purpose. Let's remove it to avoid any confusion!)

[Rekkonnect]

Most code editors have a margin on the line number panel to make it more readable and prettier. Removing the margin feels much worse to me.

[mgarstenauer]

@Rekkonnect Thanks for the feedback. I was mainly worried about the margins on the other sides. Having space between the scrollbars and the window, felt like a bug.

I pushed a small change. Keep some margin on the left for readability, remove the top/right/bottom margins. Here is what it looks like:

[mgarstenauer]

I increased the minimum size of the LineNumberMargin. This ensures that there is enough space between the line numbers and any other text editor margins.

Together with the breakpoint margin it looks like this:

[mgarstenauer]

I have added a property LineNumberMargin.MinWidthInDigits. The width of the LineNumberMargin is now styleable from XAML. Example:

<AvalonEdit:TextEditor.Styles>
  <Style Selector="editing|LineNumberMargin">
    <Setter Property="MinWidthInDigits" Value="4" />
  </Style>
</AvalonEdit:TextEditor.Styles>

The defaults are as before to make sure that the changes in this PR don't mess up the layout of existing apps.