[Inspector] Change view selector to drop-down instead of buttons

Issue nasa/mct#135 specifies that view-switching buttons in the Inspector should be replaced by a drop down menu, which allows the name of the view to be seen at all times. This will also better-handle situations where the number of views available is high. A related change is the addition of a "gear" icon for expanding/collapsing a view's control area, instead of the "twistie" arrow

Changes include:
* Introduce the "switcher" as its own view and view type. This facilitates its re-use in other inspectors (Panel Inspector and Timeline Inspector are both plugin-provided) and isolates its code from the Inspector itself.
* Add new view icons
* Modify code for "Save / Save All / Discard" prompt such that the platform's WindowManager is utilized (reduces coupling with Swing)
* Replace twistie with gear button

 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Code review by pull request
 Describe potential impact on other areas of the code: Inspector, custom inspectors
 Describe any test prerequisites for this fix (e.g., restart the database): N/A
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N