Dockable panels: Improve contrast of the titlebar area

When the Play and Mixer panels are docked in stacked mode with the Inspector, it is difficult to see where the panel begins and the Inspector ends. This because the title of the panel is in plain text with no Bold and no background colour. This is unsettling, especially if you are unfamilar with the geography of the sidebar; it also makes it harder to locate the titlebar area in order to undock the panel;

Suggestion: Improve the contrast of the titlebar of the panel so that it is clearly visible. For example, embolden the panel name and give it a background colour.

See also: #292271: Inspector: make section headers explicitly full width to prevent accidental closure/opening .