Enable UrsaWindow and Dialog border customization.

[rabbitism]

in some environment (well ubuntu), borderless window is not possible. So it is necessary to expose border customization for user. User can now globally add border brush and border thickness to UrsaWindow, DialogWindow, DefaultDialogWindow and MesageBox.

[rabbitism]

User can globally define UrsaWindow/Dialog/MessageBox border as following:

        <Style Selector="u|DefaultDialogWindow"> <!-- same for u|UrsaWindow, u|DialogWindow, u|MessageBoxWindow -->
            <Setter Property="BorderThickness">
                <OnPlatform>
                    <OnPlatform.Linux>
                        <Thickness>0</Thickness>
                    </OnPlatform.Linux>
                    <OnPlatform.Default>
                        <Thickness>10</Thickness>
                    </OnPlatform.Default>
                </OnPlatform>
            </Setter>
            <Setter Property="BorderBrush">
                <OnPlatform>
                    <OnPlatform.Linux>
                        <SolidColorBrush Color="Green"></SolidColorBrush>
                    </OnPlatform.Linux>
                    <OnPlatform.Default>
                        <SolidColorBrush Color="Red"></SolidColorBrush>
                    </OnPlatform.Default>
                </OnPlatform>
            </Setter>
        </Style>