remove "using namespace std" from header files

"using namespace std" in headers is considered bad practice, so move it
either into the respective cpp files or (for files which have lots of
other std::xxx usage anyway) just write it out explicitly.
Looking at the headers and the affected cpp files, one can actually see
why it is bad practice, as it's spreading very far ;-)

src/gui/update.cpp

@@ -411,7 +411,7 @@ bool CFlashUpdate::checkVersion4Update()
 		if (allow_flash)
 			UpdatesFilter.addFilter(FILEBROWSER_UPDATE_FILTER);
 
-		string filters[] = {"bin", "txt", "opk", "ipk"};
+		std::string filters[] = {"bin", "txt", "opk", "ipk"};
 		for(size_t i=0; i<sizeof(filters)/sizeof(filters[0]) ;i++)
 			UpdatesFilter.addFilter(filters[i]);