TabControl Underlined TabPanel

[punker76]

What changed?

This PR adds a new attached property to add a new underlined type for the TabControl and MetroTabControl.

• Add new Underlined attached property to TabControlHelper. This property controls the type of the underline type which can be TabItem or TabPanel (with TabItem).

    /// <summary>
    /// Specifies the underline position of a TabControl.
    /// </summary>
    public enum UnderlinedType
    {
        None,
        TabItems,
        SelectedTabItem,
        TabPanel
    }

• Make IsUnderlined obsolete
• Refactor Underline control, cause it doesn't work as expected (Bottom and Right doesn't works nicely)
• Add new Brush attached properties to enable easy changing the underline brushes

Controls:TabControlHelper.UnderlineBrush
Controls:TabControlHelper.UnderlineSelectedBrush
Controls:TabControlHelper.UnderlineMouseOverBrush
Controls:TabControlHelper.UnderlineMouseOverSelectedBrush

Closes #2895 [RFC] [Enhancement] Proposed TabControlHelper.IsUnderlined Change

[amkuchta]

Could we add one more item to the enum:

public enum UnderlinedType
{
    None,
    AllTabItems,
    SelectedTabItem,
    TabPanel
}

where SelectedTabItem performs like TabPanel but without the line all the way across the TabPanel? I can implement, if you'd like

[amkuchta]

@punker76 👍 this is gorgeous!!

[thoemmi]

LGTM 👍

[amkuchta]

HEAVY BREATHING

[amkuchta]

@punker76 Was this designed to work with MetroAnitmatedTabControl and MetroAnimatedSingleRowTabControl? The underlines do not render on either control...

[punker76]

@amkuchta it's coming...

[amkuchta]

@punker76 awesome, no rush. I just didn't know if it was supposed to work or not, and didn't want to open an issue without checking in first. These look great!