[GTK] if tabs go outside of content, there should be arrow to see available tabs

[Miepee]

Expected Behavior

If all the tabs don't fit on screen, there should be arrows to switch between the available tabs

Actual Behavior

The content just switches further

Steps to Reproduce the Problem

1. Try the TabControl test on Eto.Test and create lots of tabs

Specifications

• Version: 2.7.3
• Platform(s): GTK
• Operating System(s): Arch Linux

[cwensley]

The native TabControl control for MacOS (NSTabView) doesn't have these features and are intended for only a few tabs, so the other platforms follow suit. Please use DocumentControl for more tabs, which has other features like closing, reordering, and having more tabs than can fit horizontally.