Fix incorrect visibility in Attach to Process when the filter is changed.

Fix incorrect visibility in Attach to Process when the filter is changed.

Previously, when the text in the filter edit box was changed such that more
processes should be visible, a seemingly random set of processes remained
visible that did not match the filter. This was obviously due to
Q[3]ListViewItem::setVisible(true), which not only makes the item itself
visible, but also makes all its child items visible (not recursively,
though). This would cancel all setVisible(false) calls that were made
on each level before the first setVisible(true) call.

To fix this, we call setVisible(true) before the search descends to the
child items (because it does not hurt that all child items become visible
implicitly), and then call only setVisible(false) when it was determined
after the recursion that the item must be invisible.

On Linux, the process hierarchy actually has two roots: init for the
user-space processes, and kthreadd for kernel threads. Previously, the code
assumed that there is always only one root. This restriction is lifted so
that the filter algorithm can also hide kernel threads that do not match
the filter.