Added custom context menu to Spark + do UI switching using it

Replaced the not-so-pretty links directly in the app's UI with a context menu item.

Note: there seems to be a race condition somewhere: when repeatedly switching from one UI to the other multiple timees, eventually the UI alternate windows stop opening/closing in a random fashion. 'this.window' in the code becomes null. This function is dev-only, though, and the first time switching always work,  so don't spend too much effort now figuring it out.
Removed a leftover old code
Merge pull request #7 from ussuri/context-menu-ui-switching

Added custom context menu to Spark + do UI switching using it