Interface is not blocked in Extension Manager when doing long operations
I've noticed that many new users (especially when trying TYPO3 out for the first time) they have issues and sometimes mess up the system when installing introduction package.
The problem is that the orange progress bar doesn't block the ui, so user
1) doesn't recognize that there is something going on
2) can click multiple times to install the same extension or to install another one when the first is still in progress.
The same issue applies to the extension search box, also it's hard to notice that something is loading.
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