avoid to force developer to set total each time

[12rambau]

I should be able to init the progress of an Alert first and then simply update the progress.

as in:

from sepal_ui import sepalwidgets as sw 

alert = sw.Alert()

# init 
alert.update_progress(0, "toto", total=10)

# loop
for i in range(10):
    alert.update_progress(i)

in the current implemetnation, total need to be set in every calls