New icons:
- for message window button (by Sloth)
- for techs Planetary Barrier Shield and System Defense Mines (by Ta'Lon)
- for not yet implemented search function (by (Ta'Lon)

git-svn-id: svn://svn.code.sf.net/p/freeorion/code/trunk@7738 dbd5520b-6a0a-0410-a553-f54834df5b05