Revert "Added drop shadow to sidebar header."

This reverts commit b734e90583fcb79b1553c9494eabe876b9d09d2a.
Improved styling of scrollable tabs.

Scroll buttons are now better aligned with tab text, have the correct
Material Design icon colour, and tab text is better vertically centered.
Disabled scroll buttons also no longer cause hovering cursors to display
pointers.
Improved display of tab scroll buttons in editor.