Add attribute to mark parts of install-tree possibly unstable

We need a way to push unstable snapshot packages to production online so that possible errors there won't affect other parts of the install-tree, so that the user can continue using the installer.

Now the installer will bail out if there is even one error anywhere in the install-tree.

e.g. in qt.59/package.xml

<Unstable>true</Unstable>

Then any error under "qt.59" would not tamper using the rest of the install-tree.