refactor: cleanup project. ready for refactor.

- remove all old code
- build directory hierarchy, add basic cmake scripts.

cmake/custom_import_virtools.cmake

@@ -0,0 +1,35 @@
+# ========== Check Variables ==========
+# Check Virtools version variable
+set(SPECIAL_VIRTOOLS_VERSIONS
+    "21"
+)
+set(LEGACY_VIRTOOLS_VERSIONS 
+    "25"
+    "30"
+    "35"
+    "40"
+    "50"
+)
+if (VIRTOOLS_VERSION IN_LIST SPECIAL_VIRTOOLS_VERSIONS)
+    message(
+        "Specified Virtools version: ${VIRTOOLS_VERSION}."
+        "Please note in this verions, your provided Virtools root path must be the root of GitHub repository: doyaGu/Virtools-SDK-2.1"
+    )
+elseif (VIRTOOLS_VERSION IN_LIST LEGACY_VIRTOOLS_VERSIONS)
+    message("Specified Virtools version: ${VIRTOOLS_VERSION}.")
+else ()
+    message(FATAL_ERROR
+        "You must set VIRTOOLS_VERSION variable to specify the version of your target Virtools before configurating this project."
+        "The valid Virtools version are 21, 25, 30, 35, 40, 50."
+    )
+endif ()
+
+# Check Virtools path variable
+if (NOT DEFINED VIRTOOLS_PATH)
+    message(FATAL_ERROR
+        "You must set VIRTOOLS_PATH variable to specify the root folder of Virtools before configurating this project."
+        "Please note this path is different when you picking Virtools 2.1 version."
+    )
+endif()
+
+# ========== Import library ==========