macosx: Fix toolbar segmented buttons bug and bring design closer to mockups

At the moment, the library window toolbar uses an older button style and smaller buttons that are different from the proposed designs. Additionally, prior to this PR, the toolbar would have a visual bug that would prevent all the buttons from being accessible:

This PR changes the style of the toolbar to more closely match the proposed designs, while also fixing the visual bug preventing the navigation buttons from being used:

The key changes visually are:

• Fixed the clipped segmented title buttons
• Increased the size of the segmented title buttons
• Moved the cone icon to the left of the navigation buttons
• Correctly centred the segmented title buttons