CMake 3.1 is the actually required version

Summary:
CXX_STANDARD is only available since 3.1.
$ git show 913394af2: cmTarget: Add CXX_STANDARD and CXX_EXTENSION...
$ git tag --contains 913394af2 | head -n: v3.1.0

3.1 also has CMakePackageConfigHelpers (available since 3.0),
allowing to drop ECMPackageConfigHelpers (it actually fixes a CMake
warning when requiring >=3.0).

Furthermore move cmake_minimum_required(VERSION 3.1 FATAL_ERROR) to
the top of CMakeLists.txt like suggested by CMake's documentation.

Test Plan: cmake .. & ninja

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7784