Allow dynamic icon selection

The Folders plugin allows the user to pick between a few icon types in the configuration. It would be nice if the multibranch pipeline plugin could follow a similar model.

Specifically, allow ball icons (for all branches or select branches), or allow a stylized multibranch icon.

• Allow ball icon or stylized multibranch icon
• Apply to all branches, or a selection of branches
• Current blue-ish icon, red-ish icon, flashing icon (based on src/images/pipelinemultibranchproject.svg)