Added quick fix and link to "Build Variants" window to variant-selection errors.

This CL adds the following hyperlinks to error messages, shown in the
"Messages" window, that are related to variant-selection conflicts:

- "Quick fix." This hyperlink will fix the conflict and update the state
  of the IDE.
- "Show conflict source in window." This hyperlink will select the source
  of the conflict in the "Build Variants" window.

Change-Id: I78420fa2be6e6da94d8807f4b3d4c1b454dd324f
10 files changed