Set focus to package when searching with key strokes

[tkohlmeier]

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Using FindDescendant() to find the selected PackageItemContainer to focus it after selecting it. Also using StartBringItemIntoView() to scroll to item.

This fixes checking the package by pressing Space, but when pressing Up or Down, the wrong package is still selected (that's why this PR is a draft).

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Closes #3355

[tkohlmeier]

Now handling PreviewKeyDown event to handle Up/Down/Home/End keys which works nicely most of the time. Sometimes, a wrong package is being selected, not sure why.

It's really sad that we have to handle such basic functionality ourselves...

[tkohlmeier]

I think I was able to solve all problems. The CharacterReceived listener had to be moved to PackageList since otherwise the search input would lose focus when entering text. Focus() is called with FocusState.Keyboard because FocusState.Programmatic is buggy and sometimes breaks PageUp/Down functionality. I also had to handle Up/Down/Home/End keys since the default handling is also buggy.

Note that I changed the scroll position from index - 3 to index since this is common behavior, but of course I can revert that if you disagree.

[marticliment]

I will test and merge as soon as I can.

Thanks!