This allows `FLEX` to be linked statically to a binary that has `-Wstrict-prototypes` enabled (for example in CocoaPods without `use_frameworks!`.
Changes:
- Apply `void` to all the empty function/block declarations that don't take arguments.
- Apply `SEL`/`UIImage *` to the others that actually take arguments.
- Remove `CLANG_WARN_STRICT_PROTOTYPES = NO` since the default is enabled.
Also add FLEXMutableListSection which wraps the collection content section in a way that makes displaying a simple list of content straightforward.
Adopt additions in appropriate view controllers.
Matt RobinsonandTanner Bennett