Fix generic wxSearchCtrl best size calculation.

Fix generic wxSearchCtrl best size calculation.

The best size of its text control part was not calculated correctly any more
because a wrong best size was cached during wxSearchTextCtrl construction,
when the final class overridden DoGetBestSize() was not called.

Fix this by explicitly invalidating the best size at the end of constructor
for now even though it would be arguably better to fix this in some way not
requiring anything extra to be done to always take the overridden method into
account, especially as it used to work before. But it's not clear how exactly
to restore this so for now do at least fix wxSearchCtrl appearance.

Closes #14708.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72577 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775