Valgrind plugin: fix for just partial usage of coreplugin/coreconstants.h values

Valgrind plugin CallgrindTool and MemcheckTool modules sometime use
coreplugin/coreconstants.h values for icons (e.g. Core::Constants::ICON_REDO),
and sometime - an own constant is used like ":core/images/prev.png".

This commit is fixing this issue and changing everything to Core::Constants
usage.

Change-Id: I8d6f473a66a3181b03e2193314d17661df1d4cc1
Reviewed-by: hjk <qthjk@ovi.com>