reduce distraction by adding placeholder entries

[UlrichScholz]

Code minings are added to the editor content as they are available, creating new, unnumbered lines. These new line move the visible content, so causing distraction.

Improvement: always add placeholder/dummy entries for code minings that are not yet avaialble. When they are, replace them by the actual minining. This way, no new lines have to be created afterwards

[angelozerr]

This project is a POC. Since a part of JDT code mining was moved to JDT, please create an issue in Eclipse bugzilla . Thanks!