Require C++17

As discussed in https://mail.kde.org/pipermail/plasma-devel/2020-August/118263.html we agreed that requiring C++17 in Plasma is fine.

There's no code that actually uses C++17 features yet, but specifying the requirement anyway has advantages.
1) It documents the policy in a much more discoverable place than a wiki page or similar could do
2) Building in 17 mode allows the compiler to do some stuff implicitly, like guranteed copy elision

CMakeLists.txt

@@ -7,6 +7,10 @@ set(PROJECT_VERSION_MAJOR 5)
 set(QT_MIN_VERSION "5.14.0")
 set(KF5_MIN_VERSION "5.72.0")
 set(INSTALL_SDDM_THEME TRUE)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
 find_package(Qt5 ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS Svg Widgets Quick QuickWidgets Concurrent Test Network)
 find_package(ECM ${KF5_MIN_VERSION} REQUIRED NO_MODULE)
 set(CMAKE_MODULE_PATH ${ECM_MODULE_PATH})