Stop Exporting Names Through Usings in the Remaining Headers

The last headers with a using or namespace alias at namespace scope:

- TCPConsole.hpp imported boost::asio::ip::tcp into Slic3r::Utils for
  two member declarations. The alias is now a private member of the
  class.
- WebSocketClient.hpp declared four namespace aliases and a tcp alias
  at global scope, each used only by the header. The names are spelled
  out.
- Repair.hpp aliased CGAL::Polygon_mesh_processing as PMP in
  Slic3r::tex2color. The three functions that use it declare the alias
  themselves.
- PreciseSeam.hpp, Thumbnails.hpp and MarchingSquares.hpp used a
  using-declaration or directive for one or two spots each; those spots
  are qualified. Thumbnails.hpp's "PNG"sv default argument becomes
  "PNG", which converts to the std::string_view parameter the same way.
- tests/sla_print/sla_test_utils.hpp had "using namespace Slic3r;" and
  tests/filament_group/fg_test_serialization.hpp "using json =
  nlohmann::json;" at global scope. The headers qualify their own names;
  the two SLA test sources get the directive themselves.

Also removed: twelve type aliases in headers that nothing references
(ConflictObjName, CircleSqf, CircleSqd, TRawBuffer, DistanceFunction,
SamePair, ExtruderNozzleInfos, Vec2dEvent, Vec2dsEvent, Vec3dEvent,
t_option, t_optgroups, Plater::fs_path) and a duplicate
fn_ft_job_msg_destroy alias in FileTransferUtils.hpp.

tests/sla_print/sla_print_tests.cpp

@@ -31,6 +31,8 @@
 #include "libslic3r/BoundingBox.hpp"
 #include "libslic3r/ExtrusionEntity.hpp"
 
+using namespace Slic3r;
+
 namespace {
 
 const char *const BELOW_PAD_TEST_OBJECTS[] = {