Make the extensions dialog non-modal and dockable

I assume all the extensions will have to reuse the same dialog.
Or can extensions can be 'chained' together if multiple extension dialogs are open at the same time ?

> I assume all the extensions will have to reuse the same dialog.

Based on my experience: no. A user may want to have two or three extension dialogs open at the same time, and use them alternately (depending on the task at hand, this can occur frequently). Currently, as long as 'Live Preview' is not checked, the floating dialogs can remain open without preventing to work on the current drawing or to use other extensions.
If the same (dockable) dialog is to be reused, IMHO it would require quicker access to other extensions without having to search them somewhere burried deeply in a sub-menu of the 'Extensions' menu (the menu gets easily crowded as soon as external custom extensions are installed).

Related (redesign/enhance extensions interface):
Proposal to redesign the extension interface (AFAIR not discussed on 'inkscape-devel' so far):
<https://blueprints.launchpad.net/inkscape/+spec/long-menu-accessibility>
Another feature direly lacking is
<https://blueprints.launchpad.net/inkscape/+spec/extensions-presets>

An increasing number of (internal) extensions - which also use the INX-based extensions dialog AFAIU - are actually in the menu 'Filters':
 <https://blueprints.launchpad.net/inkscape/+spec/custom-predefined-filters>