Ability to set the gutter on CustomDialog

[gnewtzie]

Is your feature request related to a problem? Please describe.
When showing a CustomDialog, I would like the ability to change the default gutter on the left and right of the content, so that i can show more content.
The existing implementation uses a fixed 25* grid column definition, hard coded in the CustomDialog template.

                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="25*" />
                    <ColumnDefinition Width="50*" />
                    <ColumnDefinition Width="25*" />
                </Grid.ColumnDefinitions>

Describe the solution you'd like
I'd like to be able to set the width weighting for the gutter from the MetroDialogSettings when passing in to the .ShowMetroDialogAsync() method. Something more like:

                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="{Binding GutterPercent}" />
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="{Binding GutterPercent}" />
                </Grid.ColumnDefinitions>

Additional context

Closed Issues

[Evangelink]

FYI I have deleted my previous post

We can actually handle this in quite a lot of ways:

• Bind each margin to a double property which represents a percentage (ratio) and use a converter to transform this value into a GridLength and fix the middle column to *. The drawback of the double property is that the user can't define a fix margin (e.g. 300).

• Bind each margin directly to a GridLength property and fix the middle column to *.

• Fix the left and right columns to * and allow to bind the middle column width (again there is the question of using double or GridLength)

• Use some kind of enumeration for the left/right margin space like NoMargin, SmallMargin, LargeMargin, honestly I don't think that's a really good idea because we are putting a useless limit to the customization.

cc @punker76

When the choice is made, I will happily create a PR for this.

[punker76]

@Evangelink #3518