WIP: add Container type with corresponding Properties configuration

The configuration of the `Container` types is very much inspired by
[clay](https://github.com/nicbarker/clay).

src/event.zig

@@ -6,9 +6,10 @@ const terminal = @import("terminal.zig");
 const Size = @import("size.zig");
 const Key = @import("key.zig");
 
-// System events available to every application.
+/// System events available to every `zterm.App`
 pub const SystemEvent = union(enum) {
     /// Initialize event, which is send once at the beginning of the event loop and before the first render loop
+    /// TODO: not sure if this is necessary or if there is an actual usecase for this - for now it will remain
     init,
     /// Quit event to signify the end of the event loop (rendering should stop afterwards)
     quit,