Default --paper-tabs-selection-bar-color to --app-header-text-color

[KTibow]

Right now, if you set the background (--app-header-background-color) to white, and the text color (--app-header-text-color) to black, the bar under the selected tab is white. Ideally it would be set to --app-header-text-color by default.

[spacegaier]

Will be fixed by PR #8227.