textblock: Allow to use textblock as a context

This PR provides an helper to the TextBlock in order to use it as a python context and stack multiple text together.

This provides some interesting features

• Reduce the verbosity
• Simplify the integration with code written as context (like in !5928 (merged))
• Do not interleave the main process with asyncio (asyncio is used on the side for the display only)

from bliss.shell.pt.text_block_app import TextBlockApplication
import gevent


## Simple example

app = TextBlockApplication()
with app.exec_context():
    tb = app.first_text_block()
    tb.set_text("Processing: Step 1", height=1)
    gevent.sleep(1)  # do some real stuffs
    tb.set_text("Processing: Step 2", height=1)
    gevent.sleep(1)  # do some real stuffs
    tb.set_text("Processing: Step 3", height=1)
    gevent.sleep(1)  # do some real stuffs
    tb.set_text("Processing was done", height=1)


## Interruption

app = TextBlockApplication()
with app.exec_context():
    tb = app.first_text_block()
    tb.set_text("Init", height=1)
    try:
        tb.set_text("Processing...", height=1)
        gevent.sleep(5)
    except:
        tb.set_text("Processing aborted: Restore...", height=1)
        try:
            gevent.sleep(3)
        except:
            tb.set_text("Processing and restoration aborted", height=1)
            raise
        tb.set_text("Processing aborted", height=1)
        raise
    gevent.sleep(1)
    tb.set_text("Terminated", height=1)