Since these changes:
* commit f1de6f6682 (cmake-gui: Support building against Qt6,
2020-12-17, v3.20.0-rc1~205^2~7)
* commit 134d69121e (cmake-gui: Restore search bar for cache view and
environment dialog, 2021-02-22, v3.20.0-rc2~12^2)
the search bar is no longer case insensitive when using Qt 5.12.[0-1],
and possibly some later versions. It works as of Qt 5.15 though, so
update our version check accordingly.
Fixes: #21884
Don't use subclasses to connect to QAbstractButton::clicked. This is
slightly dodgy, but of more immediate importance, tweaking it will allow
us to change the widget type of some QToolButton instances that should
be QPushButton.
