Convert QGLWidget usage to QOpenGLWidget

[dsboger-zz]

QOpenGLWidget is the modern replacement and has several advantages. In
this case, it fixes #86.

The only big change needed was replacing renderText calls with
QPainter::drawText ones.

[dsboger-zz]

Some documentation. QOpenGLWidget was introduced in Qt 5.4 (dec/2014) so all currently supported versions of Qt (5.6 LTS, 5.9 LTS and 5.10) should work.

EDIT: I mean supported by Qt upstream.

[gillesdegottex]

Great! Thanks!

[dsboger-zz]

I think I know why the travis checks on PRs are passing when they should not. It seems they are using master branch and not the PR branch.

[gillesdegottex]

Indeed:

$ git checkout $TRAVIS_BRANCH
Warning: you are leaving 2 commits behind, not connected to
any of your branches:
  272f5e1 Merge 56f615d2389503d0e765da2f42d439bcbe350ebc into b8e3d8f7d110d5ae891576c99dd99e9610017cf1
  56f615d Convert QGLWidget usage to QOpenGLWidget
If you want to keep them by creating a new branch, this may be a good time
to do so with:
 git branch <new-branch-name> 272f5e1
Switched to branch 'master'
Your branch is up to date with 'origin/master'.

[gillesdegottex]

Ubuntu 18.04 is out now, it seems it was time to switch to xenial builts anyway.

It seems fixed now.

[gillesdegottex]

The issue with non-properly checkout PR is still here though.