* src/*/CMakeLists.txt: adds SYSTEM to arget_include_directories()
* src/*/CMakeLists.txt: removes duplicate sources from lisbslic3r_sources and SLIC3R_GUI_SOURCES"
* .gititnore: adds CMakeLists.txt.user and CMakeLists.txt.autosave
* deps_src/*/CMakeLists.txt: adds SYSTEM to arget_include_directories()
* removes #pragma once from .cpp file
Move many third-party components' source codes from the src folder to a new folder called deps_src. The goal is to make the code structure clearer and easier to navigate.
