[client] Debug: dump GUI memory profiles to /tmp/nbgui

Temporary debug patch for the GUI memory consumption investigation. Writes
two snapshots - one at startup, one after 5 minutes - each into its own
/tmp/nbgui/<timestamp>-<pid> directory containing the heap, goroutine and
threadcreate profiles plus a memstats.txt summary.

client/ui/main.go

@@ -80,6 +80,11 @@ func init() {
 
 func main() {
 	daemonAddr, userSetLogFile := parseFlagsAndInitLog()
+
+	// Debug patch, not for release: dumps heap/goroutine profiles to
+	// /tmp/nbgui for the memory consumption investigation.
+	startMemProfiler()
+
 	conn := NewConn(daemonAddr)
 
 	// Without --log-file, the GUI manages a gui-client.log that follows the