ControlsHelper split up

Fixes #1520 

Two new classes `TabControlHelper` and `GroupBoxHelper`.

All properties here mentioned as had been moved had their dependencies moved too with the respective name changes (if applicable).

## GroupBox ##
`MahApps.Metro.Controls.ControlsHelper.GroupBoxHeaderForegroundProperty`
__moved to__
`MahApps.Metro.Controls.GroupBoxHelper.HeaderForegroundProperty`

## TabControl ##
`MahApps.Metro.Controls.ControlsHelper.IsUnderlinedProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.IsUnderlinedProperty`

`MahApps.Metro.Controls.ControlsHelper.HeaderFontSizeProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.HeaderFontSizeProperty`

`MahApps.Metro.Controls.ControlsHelper.HeaderFontStretchProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.HeaderFontStretchProperty`

`MahApps.Metro.Controls.ControlsHelper.HeaderFontWeightProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.HeaderFontWeightProperty`

`MahApps.Metro.Controls.ControlsHelper.VerticalScrollBarOnLeftSideProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.VerticalScrollBarOnLeftSideProperty`

`MahApps.Metro.Controls.ControlsHelper.TransitionProperty`
__moved to__
`MahApps.Metro.Controls.TabControlHelper.TransitionProperty`