Fix box sizer layout algorithm to respect both proportions and min sizes.

Fix box sizer layout algorithm to respect both proportions and min sizes.

The new version of the algorithm tries to distribute the entire space
allocated to the sizer among its children, just as the version in 2.8 did,
while still respecting minimal children sizes first and foremost. This means
that the space allocated to the item will always be at least its minimal size
if the total space is at least equal to the sum of minimal sizes of the
children but that if there is enough space, the proportions will be respected
too.

Extended the unit test to check that laying out various combinations of three
elements results in the expected results.

Closes #11311.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63706 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775