Implement default width and height for Dockview panels

[qcz]

When adding a new panel to the top/right/bottom/left of a panel in a Dockview component, the available space is evenly distributed between the two panels. This is not always a desirable default. For example if I have a narrow tree on the left and a list on right of the center panel, I do not want them to take 1/3-1/3 of the screen. I know I can resize them, but it has its own weird behavior (see #567).

I see that the underlying GridView can handle fixed widths/heights when adding panels.

It should be nice if we could define default widths and/or heights for panels. Default width would be used for the width of a panel when adding to left or right of a panel, and the default height would be used when adding a panel to top or bottom of a panel. Otherwise it would be ignored.

I am willing to try to implement this if the project is interested in this feature.