extensiongnomegnome-shell-extensionunity
Today I saw an article about extensions for GNOME Shell, so it got me thinking a couple of questions that may apply to many users in Unity:
Will the GNOME Shell extensions be compatible with Unity?
Can I install them directly to Unity from the https://extensions.gnome.org/ website?
In case 2 is NO, is there a similar idea for Unity in the works?
You're actually looking at two separate applications. The screenshot at the top is system-config-printer (called "Printing" in the shell) and the screenshot at the bottom is gnome-printers-panel (called "Printers" -- but they may have the same name in your language!). Both are available in both desktop environments.
The latter one is probably supposed to replace the former in Gnome 3, but as you have noticed, it isn't as functional as the old one (yet). That explains why both are installed.
I can't explain why printing doesn't work in Gnome Shell. Do you still experience problems if you use system-config-printer to set up your printer in Gnome Shell?
The error message you are reporting with gnome-tweak-tool occurs when either logging into Gnome-Classic or if logging into Gnome-Shell - but it doesnt allow a full Gnome-Shell GUI but reverts to Gnome-fallback.
From your latest edit - this is the case - the screenshot shows that you are using gnome-fallback. This occurs when 3D acceleration fails.
N.B. Gnome-Shell should look like:
To get the full Gnome-Shell GUI, check that your virtualbox settings have 3D acceleration checked, 128MB VGA RAM and that your host graphics supports 3D Hardware acceleration.
Best Answer
No. They are only for gnome-shell. They are expecting a special API, are written in javascript, and in general are too different to be used on Unity.
Note that the idea of writing most of the gnome-shell UI in javascript will make it very easy to write extensions that can hook near everywhere. Of course it will take a penalty in speed. It's quite similar to the trade-offs between Firefox and Chrome. Firefox UI is written in XUL (an extension of javascript) and Chrome in C++. The former is way easier to extend, but the later is likely going to be always faster and more responsive (I'm talking about the UI, not the rendering or javascript engine).
So to answer you second question, no, likely Gnome-Shell is going to be always more customizable than Unity as a result of a design decision.