[GUI] Show loading indicator when executing AJAX requests.

[SchizoDuckie]

I'm executing an alter table query right now that adds indexes on a table. Naturally, I forgot that this table already has 7 million rows, and now i get now UI feedback that the process is still running.

maybe add an optional warning before starting 'add index' queries here?
You don't want to do this accidentally on production, most likely...

[victorstanciu]

Good point, it shall be done.