Do not clear filter field and results when switching to the main panel

[Mingun]

If you open filter panel (Ctrl+F), paste something to the filter textbox, switch back (Ctrl+Shift+H) (for example, to refine something from the commit message) and switch to the filter panel again, filter is cleared.

Expected behaviour: filter term is not cleared, filtered results are preserved.

[love-linger]

Done.