Standard text block

This PR feature a standard function for the textblock.

For now this code have to be called from the eval greenlet, in order to constrain the way it is used (this enforce a strict DAG of calls, which is easy to handle for now). This could be released in the future.

with text_block() as tb:
    tb.set_text("Processing...")
    sleep(5)
    with text_block() as tb2:
        tb2.set_text("Processing2...")
        sleep(1)
    sleep(5)
    tb.set_text("Done")