Panel tabs usability improvements

Proposal

Split "panel tab options" from "new tab menu".
Add a "plus" button next to the open tabs, to better communicate the action of adding a new tab to the row.
Remove (or give users the option to remove) the "close" icon.
- Closing tabs would be possible by right clicking the tab, or in a more conservative approach, by keeping the down chevron button.
The mockup below also includes additional options for docking tabs, from @mkov Merge Request inkscape!6922 (closed)
- This illustrates the possibilities unlocked by splitting "tab options" from "new tab menu".
- In case new specific controls for active tabs show up, they can be better accommodated in a dedicated menu.

Active tabs display a "close" button, which makes makes the tabs "dance" and change horizontal position when you switch between them. That can compromise muscle memory, and users may need to always pay extra attention on what tab they are clicking, possibly interrupting their concentration.
Options for the open tabs (Close Current Tab, Close Panel, Move Tab to New Window) reside in the same dropdown menu that displays all the other panels to be opened.
Having a centralized place to show all possible panels is good for feature discoverability and also for easy access. But these options are not directly related to the open tab options, and they take too much space.

There's no "Close tab" button on the tabs. Users need to right click the tab and select "Delete". (behavior not displayed in reference image below)
To open a new tab, there's a "plus" button that shows the options.

There's a single button on the right corner of the panel header. It usually contains only "close" and "close panel group" options.
When certain tabs are active, like "Colour", it displays additional options related to that tab.
There's no close button for each tab individually.
There's no way of adding new tabs to the panel from inside of it, and users need to use app menus to open additional panels.

*Other key competitors like Corel Draw and Adobe Illustrator were not tested. Screenshots and references are welcome.