Use Left/Right Arrow keys to faster move over threaded messages list (#6399)

Fixed some inconsistencies in list widget expand/collapse code.

Made the arrow keys behavior like in Thunderbird, i.e.:

• Right Arrow expands thread or selects closest child if already expanded,
• Left Arrow collapses thread or jumps to closest parent,
• Left Arrow + Ctrl selects top-most parent,
• Up/Down Arrow keys behavior didn't change.