Tool window icons should be monochrome when selected

[kenzieschmoll]

The Flutter icon is not visible when the tool window is selected because it blends into the blue background. When selected we should use a version of the icon that is monochrome.

[c5inco]

See this comment: #7574 (comment)

Basically, there is no selected state icon. The colors used in the SVG have to match the IntelliJ icon color palette as it swaps colors at runtime by detecting the colors in the SVG itself to white.