Style: Partially apply clang-tidy's cppcoreguidelines-pro-type-member-init

Didn't commit all the changes where it wants to initialize a struct
with `{}`. Should be reviewed in a separate PR.

Option `IgnoreArrays` enabled for now to be conservative, can be
disabled to see if it proposes more useful changes.

Also fixed manually a handful of other missing initializations / moved
some from constructors.

editor/plugins/skeleton_3d_editor_plugin.h

@@ -131,7 +131,7 @@ class Skeleton3DEditor : public VBoxContainer {
 
 	EditorFileDialog *file_dialog = nullptr;
 
-	bool keyable;
+	bool keyable = false;
 
 	static Skeleton3DEditor *singleton;