Migrate from copies from external plugintemplate to git submodules (#223)

* Add plugintemplate module

* Remove plugintemplate sources copies

* Replace copies from plugintemplate with .gitmodules dependency

* fix ExternalIncludePath

* Revert TreeHandler.h ClCompile <-> ClInclude

Author: AntonDevil

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "external/googletest"]
 	path = external/googletest
 	url = https://github.com/google/googletest.git
+[submodule "external/plugintemplate"]
+	path = external/plugintemplate
+	url = https://github.com/npp-plugins/plugintemplate