Fix generic wxSearchCtrl size/layout code.

Override DoGetBestClientSize() instead of DoGetBestSize(), as we're really
computing just the size of our contents and like this we don't need to hard
code platform-dependent border sizes in this control itself.

Also use the client size in LayoutControls() for the same reason. This also
makes it unnecessary to pass it the width and height as it can find them on
its own. And x and y were never used in the first place, so remove them too.

Finally, center the bitmaps vertically.

Closes #16422.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77083 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775