shell: Add EvFindSidebar widget to show search results
see https://gitlab.gnome.org/GNOME/evince/-/commit/940ce8048e4fc6503dca03c1ce090ff4a11dface

eggfindbar: don't close the findbar when it loses the focus
see https://gitlab.gnome.org/GNOME/evince/-/commit/a2d781fb124bd19861035fd41dacc976e00d19ef

shell: Add the findbar to the main box instead of the view box

libview: Add ev_view_find_set_result

* Properly require WebKit for the EPub backend, instead of it being an
  optional dependency.
* Merge EPub tests together to simplify things.
* Make MathJax optional, as it always has been. If missing, MathML will
  likely not display properly, but it's probably not that big of a deal
  as it's not the only use case for the EPub backend. If not found at
  setup time, use the historical default as fallback, in case it gets
  installed later.

Note that all this MathJax stuff is currently unused, because the code
has not been adjusted to make use of this check's results.

This allows enabling the backend automatically depending on whether the
dependencies are satisfied or not, like Autotool's counterpart.

By default, the PDF backend is force-enabled and the Pixbuf backend is
disabled, as with Autotools.

pkg-config input files set -latrilview etc. but the library outputs
created were libview.so, so any project that used pkg-config was broken,
not to mention that the file itself also got renamed from its previous
value and pkg-config wouldn't find it anymore.

A recent update to libwebkitgtk2gtk-4.1 breaks using WebKitWebView
in a GtkOffscreenWindow.

Disable the thumbnail sidebar with epub documents, and disable
updating the GtkWindow thumbnail.

ref:
https://github.com/WebKit/WebKit/commit/8d7385fad3ffb31d0c438e6b6a50dffb4390840c

Fixes: #631.

Signed-off-by: Oz Tiram <oz.tiram@gmail.com>

Signed-off-by: Oz Tiram <oz.tiram@gmail.com>