Add support for _heif in imageformats plugin

[karlch]

Add support for _heif in imageformats plugin. To activate, add imageformats = heif to the [PLUGINS] section of your vimiv.conf:

[PLUGINS]
imageformats = heif

This requires a Qt extension to work:
https://github.com/jakar/qt-heif-image-plugin

[dr-carlos]

Any updates on this PR?

[jcjgraf]

Sorry, this has slipped my attention. Looks good to me 👍

EDIT
But you also need to list it in the plugins listing:

diff --git a/docs/documentation/configuration/plugins.rst b/docs/documentation/configuration/plugins.rst
index 7eac20fa..2455cbc4 100644
--- a/docs/documentation/configuration/plugins.rst
+++ b/docs/documentation/configuration/plugins.rst
@@ -47,3 +47,4 @@ Currently the following formats are supported:
 
 * cr2 (requires `qt raw <https://gitlab.com/mardy/qtraw>`_)
 * avif (requires `qt-avif-image-plugin <https://github.com/novomesk/qt-avif-image-plugin>`_)
+* heif (requires `qt-heif-image-plugin <https://github.com/jakar/qt-heif-image-plugin>`_)

And also in the changelog