Provide a mechanism to disable QR code plugins

[David Planella]

While focusing on the Touch version of Qreator, the desktop version will be kept in maintenance mode (i.e. bugfixes and a very limited scope for new features).

However, now that we've got a stable version, I'd like to see it widely used. I'm working to get it packaged for Ubuntu (see bug 1178202), but it should be entirely possible to package it for other distros, as as far I'm aware, the only distro-specific feature without fallback we've got right now is the Ubuntu Software Center QR code type.

I'd like to disable the USC QR type for Debian and Fedora, but rather than relying on a distro patch (e.g. removing the USC plugin files from the package), I believe we could do it in a relatively simple and elegant way by detecting the distro and adding a switch to disable plugins on demand.