Add support for priorization or calculation via a hook when distributing available space in DockView

[qcz]

When removing a panel from a row or column of panels, the remaining space is always equally redistributed between all the remaining panels. It is not always desirable, because the user may have perfectly configured layout with a left side list and a central panel, and just opened a side panel for a moment. After closing the side panel, Dockview unnecessarily resizes the list on the left instead of giving the space back to the central panel (which came from the central panel originally when opening the panel).

It would be nice if we could somehow specify which panels should get the newly freed space and in which ratio.

For example:

• The central panel should get all the space.
• The central panel should get 2-3x the free space than other open panels in that row/column.