unified find_package for Qt4 and Qt5

[rhaschke]

As the tutorial probably will serve as a template for many other plugins, I suggest this more uniform find_package usage for Qt4 and Qt5.
Also, shouldn't we insist in the very same Qt version, i.e. using EXACT?

[rhaschke]

The Travis error seems to be unrelated to my changes: rosdep cannot install libqt5widgets5:

executing command [sudo -H apt-get install -y libqt5widgets5]
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package libqt5widgets5

[wjwwood]

Yeah, I'll have to fix the CI.

[wjwwood]

@rhaschke can you rebase this so the CI is fixed?

[rhaschke]

@wjwwood Rebased. CI successfully built.

[wjwwood]

Thanks.