When enabled links are not allowed to change the current zoom mode and
level.

Fixes #712

Backported from https://gitlab.gnome.org/GNOME/evince/-/commit/5d8ef0fa

In dual pane mode, with a mix of landscape and portrait pages, the wrong
zoom level was getting applied to the wrong panes.

For comic book spreads with the right side page being double-wide, this
was ok. But when the left-page was double-wide, the extra zoom was
applied to the *preceeding* dual pane view (making it extra small) and
the actual wider pane got the standard zoom factor applied.

Add debug logging showing the decision being made, which uncoveres the
bug.

On HiDPI displays, rendered pages use more memory than the cache budget
expects because the size calculation does not account for the device
scale factor. This causes the cache to use up to 4x more memory than
configured on 2x HiDPI displays.

If the current zoom level was within one zoom factor of the limit, it
was not possible to scroll towards the limit. This made smooth scrolling
near the limit awkward, as unless the scroll event had a large delta it
was impossible to reach the zoom limit. Non-smooth scrolling was also
affected, but it was just much more difficult to trigger.

Fix this by allowing zooming while the current zoom level is within one
zoom factor of the limit. Add a new ev_view_can_zoom() function to make
zooming by a factor (as with smooth scrolling) more convenient.

Backported from https://gitlab.gnome.org/GNOME/evince/-/commit/6d299b69

This simplifies the logic required to enable zoom actions for
consumers of the view.

Backported from https://gitlab.gnome.org/GNOME/evince/-/commit/bbfbf913

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.