Add support for statically linking gtk4 libraries

[Hammerklavier-cn]

GTK is definitely a spendid cross-platform GUI with beautiful widgets and components. Sadly gtk4-rs does not support statically linking the GTK C libraries, which causes much difficulties on distributing on platforms other than Linux. We have to either copying shared libraries or ask users to have msys2 installed on Windows platform, for example. #563 . Yet the shared libraries consumes much space, making it a penalty compared to other rust GUIs. Recently I noticed that the rusqlite project supports linking sqlite library statically with bundle feature flag. Hope that this will be supported by gtk4-rs later.

[sdroege]

GTK4 itself does not support static linking, so that would have to be solved first.