build: clear 7 warning categories across 26 sites (#15615)

* build: clear 2 warnings - cast the NSTextField the class check already proved

mainframe_text_field is NSTextField* and was assigned a bare NSView*, which
Clang reports as -Wincompatible-pointer-types. Both assignments sit inside
if ([viewObject class] == [NSTextField self]), so the runtime type is already
guaranteed, and the line above the second one casts the same variable the same
way to call setTextColor. macOS only, since nothing else compiles this file.

* build: clear 6 warning categories from the clang-cl inventory

-Wmissing-braces (9). Aggregates whose first member is itself an aggregate.
GUID's fourth member is BYTE[8], so the trailing eight bytes take their own
braces. The others were reaching for zero-initialization with {0} and say {}
now. bbs_3mf's backup Task ends in an anonymous union, which needs braces of
its own; those braces initialize the union's first member rather than the one
named at the call site, so the RemoveBackup site says so in a comment.

-Wmacro-redefined (11). SendMultiMachinePage.hpp defines five names that
Preferences.hpp, PresetBundleDialog.hpp, ExportPresetBundleDialog.hpp and
TroubleshootDialog.hpp also define with different values, so the value in
force depended on include order. All nine of this file's DESIGN_ macros take
the SEND_ prefix it already uses for its own macros, values unchanged, so a
DESIGN_ name added elsewhere later cannot collide with it again. They read as
one page-local palette, a 900 to 400 gray ramp plus sizes, so the four with
no current readers stay: dropping them would leave gaps in a named scale. test_marchingsquares.cpp defines NOMINMAX,
which libslic3r already passes as a PUBLIC compile definition, so it takes
the #ifndef guard the other suites use.

-Wbraced-scalar-init (3). Two PushStyleVar calls resolve to the float
overload, so the braces were initializing a scalar. ConfigOptionFloatsNullable
already takes an initializer_list, so the inner braces did the same thing.

-Wmicrosoft-goto (2). Both gotos in copy_file_gui jump forward over the
initialization of size, dwRead and dwWrite, which only MSVC accepts. Those
declarations move up to join the others at the top of the function.

-Wunused-private-field (3). Every use of ColourPicker's m_clrData and
m_picker_widget is behind !defined(__linux__), so on Linux they are written
and never read; the members now carry the same guard. ParamsPanel's
m_size_move is read nowhere. Tab has its own, which is the one Tab.cpp uses.

-Wnonportable-include-path (2). BaseException.h asked for "stackwalker.h"
and the file on disk is StackWalker.h.

tests/libslic3r/test_marchingsquares.cpp

@@ -1,4 +1,6 @@
+#ifndef NOMINMAX
 #define NOMINMAX
+#endif
 
 #include <catch2/catch_all.hpp>
 #include "test_utils.hpp"