LINT: add clang-tidy step (#6295)

* Implement new travis clang-tidy build step

* This step enable some rules and enforce one rule as error
* This permits to have some C++ quality rules based on clang & clang contributor guidelines

* Fix clang-tidy reported problems on push_back -> emplace_back

src/gui/guiEditBoxWithScrollbar.cpp

@@ -743,7 +743,7 @@ void GUIEditBoxWithScrollBar::draw()
 				if (m_passwordbox) {
 					if (m_broken_text.size() != 1) {
 						m_broken_text.clear();
-						m_broken_text.push_back(core::stringw());
+						m_broken_text.emplace_back();
 					}
 					if (m_broken_text[0].size() != Text.size()){
 						m_broken_text[0] = Text;