Require C++14

Summary:
KWin already used C++14 constructs in a conditional way. This doesn't
make much sense today, it's better to just require C++14.

For KWin only gcc and clang are currently compilers of relevance. Gcc
supports C++14 since version 5 and defaults to C++14 since 6.1 [1].
Clang supports C++14 since version 3.4 [2].

An overview of compiler support in various distributions:
* Debian stable (stretch): gcc 6.3, clang 3.8
* Debian oldstable (jessie): 4.9, clang 3.5
* Ubuntu 17.04: gcc 6.1, clang 3.8
* Ubuntu 16.04: gcc 5.3, clang 3.8
* openSUSE Tumbleweed: gcc 7.1, clang 4.0
* openSUSE Leap 42.3: gcc ?, clang ? [3]
* FreeBSD: clang >= 34 in ports
* Slackware 14.2: gcc 5.3

This overview shows that every distro out there has at least one
supported compiler which can still compile KWin with this change.

[1] https://gcc.gnu.org/projects/cxx-status.html#cxx14
[2] https://clang.llvm.org/cxx_status
[3] Sorry I fail to understand openSUSE's package repository.
    It seems that there is gcc 7 available, but gcc package is 4.8

Test Plan: Compiles on my neon system

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6634