Add option to prevent software-manager to start searching before "return"key is hit

The software-manager starts searching as soon as a number of characters has been entered. The intention is to yield faster results. On less performant systems however, the searching-thread blocks the entry of further characters. As the user doesn't see the last characters reflected in the entry-field, (s)he will retype the characters. At first to no avail, abut after a while as the searching-thread is yielding cpu, all the entered characters are shown in the entry-field. As a result, the given package can't be found and the user has to re-enter the characters. So the intended gain in time results in a loss of time. If it is not possible to
lower the cpu-greed of the searching-thread, then it would be nice to have an option that the searching is started after the "return" or "enter"-key has been hit. Entering keys that are not refected into the entry-field causes a lot of frustation.