Persist size

[omgtehlion]

I propose several changes, please let me know if these are too much.

• use initializer-list to popultate QSet given to OrlojView::hideChildren, this cleans up code a little bit
• use setVisible() instead of show()/hide(), this function checks current state and is no-op if visibility does not change
• use QList instead of QSet, to easily iterate over given collection. For small lists performance might be even improved due to denser memory layout and sequential access in contains() method
• actually save size of left panel, if there were two already. Right panel takes up remaining space