Postbacks inside postbacks

[tomasherceg]

During the execution of a postback or static command, another postback/static command can be invoked (either by a postback handler or from a JS function...).

There are several combinations that can happen (postback inside postback, postback inside a static command, and so on).

We need to further investigate it. Based on what we find, we should:

• make sure it works and is covered by tests
• disable the particular combinations if they cannot be supported