build: clear 53 warnings - discarded values and i18n markers (#15421)

build: clear 53 unused value warnings

49 of them are deliberate i18n markers. L(s) expands to s, so
L("Main Extruder"); is a string literal as a statement and its value is
discarded. The strings have to stay, because the real values come from
printers/*.json at runtime and xgettext cannot scan those. Each block is
now a static const char *const markers[], which uses the values rather
than discarding them. Extraction is unchanged: the xgettext invocation
from scripts/run_gettext.bat gives 76 msgids over the two marker files
before and after, with identical msgid and msgctxt sets.

The other 4 are statements with no effect. AMSItem.cpp:117 and :174
construct and drop a wxColour(255, 255, 255); AMS_TRAY_DEFAULT_COL is
that colour, and the line above already assigns it. UpgradePanel.cpp:865
reads a member and drops it.

wgtDeviceNozzleSelect.cpp:269 writes
if (item; auto ptr = m_nozzle_rack.lock()), which puts the null check in
the init-statement position where its value is discarded, so the check
never runs, and sGetNozzlePosId then dereferences item. Nothing reaches
that today, because the only sender of the event sets itself as the
event object and the dynamic_cast always succeeds. The check now runs.

src/slic3r/GUI/UpgradePanel.cpp

@@ -862,7 +862,6 @@ void MachineInfoPanel::update_ams_ext(MachineObject *obj)
             if (new_extra_ams_ver != obj->new_ver_list.end())
                 has_new_version = true;
 
-            extra_ams_it->second.sw_new_ver;
             if (has_new_version) {
                 m_extra_ams_panel->m_ams_new_version_img->Show();
                 ver_text = new_extra_ams_ver->second.sw_ver;