Investigate adding Qt5

Context

Qt5 is a fairly widely used toolkit that currently is only provide by the KDE runtime which is large and has shorter support cycles.

Description

As a test I have built Qt5 on top of 18.08:

id: org.freedesktop.Platform.Qt5
runtime: org.freedesktop.Platform
runtime-version: '18.08'
sdk: org.freedesktop.Sdk
cleanup:
  - /bin
  - /include
  - /mkspecs
  - /doc
build-options:
  cflags: -02 -g -fstack-protector-strong -D_FORTIFY_SOURCE=2
  cxxflags: -O2 -g -fstack-protector-strong -D_FORTIFY_SOURCE=2
  ldflags: -fstack-protector-strong -Wl,-z,relro,-z,now
modules:
  - name: qt5-qtbase
    config-opts:
      - -confirm-license 
      - -opensource
      - -shared
      - -platform
      - linux-g++
      - -optimized-qmake
      - -nomake
      - examples
      - -nomake
      - tests
      - -system-harfbuzz
      - -system-sqlite
      - -accessibility
      - -dbus-linked
      - -fontconfig
      - -glib
      - -icu
      - -openssl-linked
      - -no-pch
      - -no-rpath
      - -no-directfb
      - -no-linuxfb
      - -no-kms
      - -no-cups
      - -system-proxies
      - -gtk
    sources:
      - type: archive
        url: https://download.qt.io/archive/qt/5.9/5.9.6/submodules/qtbase-opensource-src-5.9.6.tar.xz
        sha256: eed620cb268b199bd83b3fc6a471c51d51e1dc2dbb5374fc97a0cc75facbe36f
      # NOTE: KDE has a handful of patches on top of this

  - name: qt5-wayland
    buildsystem: qmake
    sources:
      - type: archive
        url: https://download.qt.io/archive/qt/5.9/5.9.6/submodules/qtwayland-opensource-src-5.9.6.tar.xz
        sha256: 6dd066b1ab518b16dc2bebc4b3629528a30161cfce48ce5d8482c67a570667d8

  # Other modules in KDE runtime:
  # qtsvg qtx11extras qtquickcontrols2 qtxmlpatterns, might be worth adding
  # qtconnectivity qtvirtualkeyboard qttools, unknown
  # libdbusmenuqt, probably valuable but not part of qt
  # qtdeclarative, is deprecated
  # qtwebchannel qtwebsockets qtwebengine, beyond scope of this
  # qtimageformats, beyond scope, bundles somes libs
  # qtscript bundles javascriptcore, probably best to skip
  # qtmultimedia qtsensors qtlocation qtserialport qtcharts qt3d qtgraphicaleffects, probably beyond scope

The resulting platform size is 44.7MB.

There are some more questions about a few more modules we may want to bundle that I noted above.
To get the answer to this probably is best done by looking at the packages on Flathub.

A big problem with just punting the optional modules to applications is every minor qt update requires a rebuild of all modules. So 5.9.7 would break applications bundling modules.