note: tools manager dialog can now be invoked only via Pluma's menu.
Preferences button in plugin manager is inactive. re-enabling it would
mean introducing even more complexity, because we would need to pass
a configure widget to libpeas, not the whole dialog.
mostly adapted from:
https://git.gnome.org/browse/gedit/commit/?id=8b6d353e0ab94ad52ce6119759db5e14b281fbba
https://git.gnome.org/browse/gedit/commit/?id=2aa6071912dac227f04f44c07e07f159b77be7ff
https://git.gnome.org/browse/gedit/commit/?id=09738cd6f5f24a37cb7dd85dfca7a5099529e5d2
https://git.gnome.org/browse/gedit/commit/?id=f08e9d2953fa213f77dce635127c3a0131fba02f
https://git.gnome.org/browse/gedit/commit/?id=d09a703e84f818dcadd7bd1c16c2091305a691c2
https://git.gnome.org/browse/gedit/commit/?id=985f3908abe3f2ade53db1364dccf0d1fd61ec1b
https://git.gnome.org/browse/gedit/commit/?id=b6086cab8c664d2cfb3e5b4a6d33c215d1b207da
https://git.gnome.org/browse/gedit/commit/?id=befd854563e4691a6b75b03695d6a7c458836312
https://git.gnome.org/browse/gedit/commit/?id=cf7549c8e5944a3fbf9d18170b6e45a552071b4c
https://git.gnome.org/browse/gedit/commit/?id=6c95d28a13ae87232346583a86ae525a22fcc651
https://git.gnome.org/browse/gedit/commit/?id=c70319048b42b898fd6c2f6ee37e98e54cea54be