Enhance backup log format and/or make it configurable

[buhtz]

This is a meta issue not fully thought thru. But we do have several demands expressed by users about the backup log (aka snapshot log).

• #1432
• #2283
• Additionally there is also the idea to make the log view more graphical with colors, icons and other fancy stuff like filters.

The issue needs more research and some decisions about re-designing core features. Also the meta-issue about the redesign of the IPC (#2260) and the replacemend of logger with Pythons own logging (#2286) need to be considered.

• It seems that BIT does catch the stdout of rsync and write it to a log. So rsyncs own logging feature is not used.
• It also might be that BIT somewhere does parse that log file or output. Changing its format might cause other problems.