build: mark missing overrides and drop unused lambda captures (1,156 clang warnings) (#15334)

* chore: mark every declaration that overrides a base virtual

clang-cl reports 42 member functions across 28 files that override a
base virtual without being marked `override`, inside classes that
already mark their other overrides. That is every occurrence of
-Winconsistent-missing-override in the tree, so the category drops to
zero and -Werror=inconsistent-missing-override becomes available as a
guard against it coming back.

Behaviour is unchanged. Each keyword goes only where clang had already
resolved the declaration to a base virtual, so it records what the
compiler already worked out and cannot affect overload resolution or
dispatch. If any of these signatures had not really overridden a base
method, the build would have failed rather than warned.

Where a declaration already carried `virtual` it is left alone and the
keyword appended, matching the surrounding declarations. Plain
`override` is used rather than the wxWidgets `wxOVERRIDE` macro, which
wx/defs.h defines as `override` beneath a comment marking it obsolete,
and which the rest of src/slic3r already avoids by 1742 occurrences to
113.

A full clang-cl build takes -Winconsistent-missing-override from 1,146
warning lines to 0. Those 42 declarations produce that many lines
because a header is re-diagnosed in every translation unit that
includes it. CalibrationWizardStartPage.hpp alone accounts for 336 of
them from 4 declarations.

* chore: drop unused lambda captures in GUI/Widgets

clang-cl reports 10 lambda captures in src/slic3r/GUI/Widgets that are
never read. Removing them changes nothing at runtime.

Every capture removed is `this` or a raw pointer. clang does not report
a capture whose type has a non-trivial destructor, since such a capture
can be held purely for its effect on an object's lifetime, so nothing
that owns or extends a lifetime is touched. The std::weak_ptr captured
beside the removed `this` in MultiNozzleSync.cpp stays.

This clears the category in GUI/Widgets only. A full clang-cl build
takes -Wunused-lambda-capture from 312 warning lines to 302, leaving
235 sites in other directories for a follow-up.

src/libslic3r/Print.hpp

@@ -930,8 +930,8 @@ public:
     // If preview_data is not null, the preview_data is filled in for the G-code visualization (not used by the command line Slic3r).
     std::string         export_gcode(const std::string& path_template, GCodeProcessorResult* result, ThumbnailsGeneratorCallback thumbnail_cb = nullptr);
     //return 0 means successful
-    int                 export_cached_data(const std::string& dir_path, bool with_space=false);
-    int                 load_cached_data(const std::string& directory);
+    int                 export_cached_data(const std::string& dir_path, bool with_space=false) override;
+    int                 load_cached_data(const std::string& directory) override;
 
     // methods for handling state
     bool                is_step_done(PrintStep step) const { return Inherited::is_step_done(step); }