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/multimesh_editor_plugin.h

@@ -46,7 +46,7 @@ class MultiMeshEditor : public Control {
 	AcceptDialog *err_dialog = nullptr;
 	MenuButton *options = nullptr;
 	MultiMeshInstance3D *_last_pp_node = nullptr;
-	bool browsing_source;
+	bool browsing_source = false;
 
 	Panel *panel = nullptr;
 	MultiMeshInstance3D *node = nullptr;