The required exiv2 version is now 0.28.x as well since it was
keeped for Qt 5 build.
Due to exiv2ConfigVersion.cmake doesn't allow us write something
like find_package(exiv2 "0.28") to require version >= 0.28.0,
we simply don't put the required version number here for now.
On macOS, now closing the window will actually hide the window
instead of quit the application, thus it will match to some other
macOS application's behavior.
Additionally, since our binary is not signed, this could avoid
seeing the request permission dialogs everytime user attempts to
open an image file inside a private place (e.g. places like the
"Downloads" folder and "access data from other apps").
