Core contains the small variants of debug run/interrupt/continue/exit
while the bigger icon variants are in projectexplorer or debugger. That
does not seem to have aany reason, at least in today's state of Qt
Creator architecture.

But above all, it stands in the way when changing debugger icons as
planned due to user feedback.

This change moves:

   Core::Icons::DEBUG_START_SMALL
to ProjectExplorer::Icons::DEBUG_START_SMALL

   Core::Icons::DEBUG_EXIT_SMALL
to Debugger::Icons::DEBUG_EXIT_SMALL

   Core::Icons::DEBUG_INTERRUPT_SMALL
to Debugger::Icons::DEBUG_INTERRUPT_SMALL

   Core::Icons::DEBUG_CONTINUE_SMALL
to Debugger::Icons::DEBUG_CONTINUE_SMALL

This change just moves icons across modules but does not change anything
in the UI.

Change-Id: I859b901c312d4d16b6c2f687395a7b48c90aab84
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>