in 0.92 extensions 2x slower to return than in 0.91

[Philippe Joyez]

Working on large files (say 5-10 Mb) I noticed that extensions are noticeably slower to return in 0.92 compared to 0.91. This happens for instance with JessyInk, Tex Text and also for another extension I wrote; For all these extensions there is the same delay after the extension has finished running its python code and before the small popup window "'extension name' working, please wait..." closes and Inkscape is useable again.

Steps to reproduce:

1- open a drawing, with several layers and many elements totaling more than 5Mb on disk.
2- if JessyInk is not yet installed in the drawing, do Extensions->JessyInk->Install/update..
3- open the Extensions->JessyInk->Effects floating window.
4- select an element in the drawing and apply a JessyInk effect (for instance "appear").

result: on my linux 64 bit machines (rnuning either Debian 8 or Ubuntu 16.04) with a ~10Mb drawing 0.91 is usable again after ~8s, while 0.92 takes ~17s to complete. During this time one cpu core is running at 100%. It's not an issue with the available RAM.

AFAICT these waiting times scale roughly proportinal to the size of the file on disk, both in 0.91 and 0.92. There does not semm to be a threshold on size, but it gets hard to properly measure differences on small files where everything goes very fast.

[su_v (suv-lp)]

Possibly related to the changes for
* Bug #967416 “inkscape crashed with SIGSEGV in file_save()”
  https://bugs.launchpad.net/inkscape/+bug/967416

(see comment 27, 29)