lib: Convert all g_print/printerr to g_debug/info/warning/critical

This logging API is more flexible, both with number of levels
and ability to use custom log handlers.
TEMP: install custom glib log handler
Merge remote-tracking branch 'origin/master' into error-reporting
Allow to send WebSocket messages without debug print
Add imgflo specific debug funcitons

We can't use glib to be able to send across to UI, because
it aborts on recursive logging, even when there is no risk of
handler being called recursively. libsoup uses g_debug statements
in their code, and we don't want to change that.
Tests: Allow to use IMGFLO_TESTS_VERBOSE to tune output
Use imgflo_debug/info/warning log functions everywhere
Don't use custom glib handler
Protocol: Send output of imgflo logging functions across