GtkSourceView: Make URIs clickable

Make GtkSourceView URIs clickable - this will allow us to create links between the books right inside the text editor.

More details on the GtkSourceView issue tracker: https://gitlab.gnome.org/GNOME/gtksourceview/-/issues/125

At some stage dar will include a facility similar to org's link abbreviations. So we will have a "catalog" file that maps all book names to their locations, like this:

...
קידושין: תלמוד/בבלי/סדר_נשים/קידושין
...
בבא קמא: תלמוד/בבלי/סדר_נזיקין/בבא_קמא
...

This will enable clean, readable and clickable links inside texts, like this: <בבא קמא:כא:ב>.

The above mentioned catalog file (optionally alphabetically sorted) will serve not only GtkSourceView, but also the humans who might want to know which books are available and where they can be found.