debug: Simplify code

Use g_parse_debug_string() instead of a handwritten variant, and use debug
flags instead of extra env variables.
Keep track of the current search result page instead of using the current page

In continous mode the page of the current search result might not the
the current page. Also make sure that when the search is restarted (the
find bar is closed and opened with the same search string) both the view
and the sidebar highlight the first match found from the current page.

https://bugzilla.gnome.org/show_bug.cgi?id=710778
shell: Do not open single page pdfs in dual mode by default

Single page pdfs will now open in full window even if the default mode
is dual mode. This will apply to pdfs that are opened for first time,
otherwise user preferences are honored.

https://bugzilla.gnome.org/show_bug.cgi?id=547098
libview: fix annotation window focus change after creation

Be able to start entering text right after selecting
or creating an annotation, also hide text cursor
when window goes unfocused (backdrop state).

https://bugzilla.gnome.org/show_bug.cgi?id=668446
djvu: Fix a segfault caused by a double free

In the case there were missing files in the djvu,
there was a double free of the file variable.

Fixes bug #710977
release: 3.11.1