Make timestamping debug output consistent

[tomaszg7]

ISSUE TYPE

• Feature Request

SUMMARY

Right now, Geeqie takes an inconsistent approach to debug messages. Some of them are timestamped by hand (like DEBUG_1("%s image done", get_exec_time());). Moreover, there is also a possibility to timestamp all debug messages in log window. It then produces messages doubly-timestamped (and with slightly different times obviously) like:

403.105885 (+00000.000003):image.c:image_complete_util:128:  403.105882 (+00000.000450) image load completed "xxx" (current)

Moreover, display of timestamp is related to debug level. It doesn't work on level 1, which is a bit inconvenient. Also, command-line switch to enable it would be handy.