DockManager: add the ability to programmatically update splitter sizes

[chris-se]

Add the ability to programmatically update splitter sizes. The user must
specify the dock area that is contained in a splitter and a list of
sizes. The list of sizes will be passed to the splitter that immediately
contains the specified dock area. If the dock area is not part of a
splitter the method will have no effect.

[nickdademo]

We did something similar in our app. The function we added was something like this: setDockLayoutSizes(const QList<QWidget*> areas, const QList<int> sizes) - allowing you to actually set the size of each splitter area for each dock area. This PR is a good start but IMO is missing the ability to know which dock area splitter size you are actually setting in setSplitterSizes().

[chris-se]

The main reason I needed this was to provide a useful initial layout -- I'm not using the method to change things after the user has been able to drag stuff around. For that ADS would require quite a bit more introspection capabilities (or the user would have to manually look at parent/child relationships of widgets). And in the initial layout case I do know what area is where, so I do know for what area I'm calling the method here.

Hence the current patch does what I need -- but I'd be happy to implement a better version if you have a useful idea for the semantics. For example, I don't quite get the semantics of the setDockLayoutSizes() you mentioned just from the function signature.

[nickdademo]

setDockLayoutSizes() is called something like this:

setDockLayoutSizes({topLeftDockArea, centralDockArea, topRightDockArea}, {80, 200, 80});

1. Then we do something like: auto *splitter = findCommonParent<QSplitter*>(areas);
2. Then for each area: auto index = getSplitterWidgetIndex(splitter, area)
3. Then we can create the QList<int> in the correct order that QSplitter::setSizes() requires.

So as you can see, there is no need to know the actual order of the areas, the function handles it.

I still think this PR is useful as it is anyway :) I just wanted to share how it could be extended perhaps.

[githubuser0xFFFF]

Thank you for your contribution.