tools: use g_print*() instead of *printf() in gst-launch

tools: use g_print*() instead of *printf() in gst-launch

We should use GLib's g_print*() functions for printing stuff in gst-launch, not printf and friends, since we're printing
translated strings, which we get in UTF-8 encoding, and GLib's print functions expect UTF-8 encoded strings whereas printf
et al. expect strings in the locale encoding, which may or may not be UTF-8.

Also add a PRINT convenience macro so we don't have to litter the code with if (!quiet) statements.