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/connections_dialog.h

@@ -111,7 +111,7 @@ private:
 	StringName signal;
 	LineEdit *dst_method = nullptr;
 	ConnectDialogBinds *cdbinds = nullptr;
-	bool edit_mode;
+	bool edit_mode = false;
 	NodePath dst_path;
 	VBoxContainer *vbc_right = nullptr;