show help content in the root controllers node in the preferences

[RJ Skerry-Ryan]

Right now the root preferences node is blank. We should provide some help content in there:

* Links to Controller forums
* Links to the Wiki for hardware compatibility
* Buttons to open the personal or system mappings folder (Bug #1296951 )
* Import/export preset? (Bug #723028 and Bug #1296952 )

What else?

[jus (jus)]

Can we make Preferences tree view resizable so we could select the correct controller in the first place? Currently, a controller name is cut off after 10 chars. Many controllers are listed multiple times because they provide multiple ports.

Also, can we take the chance to fine-tune the wording?
If no forum/ wiki link is found, we print "Support: No support available". Some users have mistakenly understood the information as if their device itself is not supported.

[jus (jus)]

* Auto expand the "Controllers" tree and a prominent help text similar to "Select a controller from the list on the left side."

[RJ Skerry-Ryan (rryan)]

https://github.com/mixxxdj/mixxx/commit/c0b9c5febceb624847980e5421ed25b0a6a9999c

I dealt with all my original points except import/export which I think should go in the individual controller page.
Tree-view resizing would be quite nice but let's do that in a different bug.
Auto-expanding is part of this commit.
No support available is fixed now.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/7367