Provide dnf plugin support to yumex-dnf

[genodeftest]

Citing @piapreawhu in [https://github.com/timlau/yumex-dnf/issues/106#issuecomment-225455827 #106]:

it would be nice if yumex-dnf made use of any enabled dnf plugins. for example, I recently installed the local plugin and was disappointed to find it only works when i use dnf from the command line, and not when i use yumex-dnf :-(

I don't know if this is possible at all, but it will be a hard task. I'm filing this as an RFE anyway, because it might be possible.

In my opinion, DNF plugins can be categorized like this:

• some plugins (like rpmconf support) are very difficult to handle since they require additional user input.
• some plugins just provide additional CLI flags. Often, it would be easier to include their functionality right into yumex-dnf (e.g. copr, show-leaves, download plugin) or it already is included (repomanage)
• some plugins really add value to dnf and are interesting to have in yumex-dnf like the local plugin.

I don't see a high priority in this and won't be working on it.

[timlau]

Most dnf plugins is extentions to the dnf cli & and cant be used by yumex-dnf, because they dont work with the DNF API alone. I will look into if some plugins can be working with yumex, but it depend alot if there is public dnf API to make it work.