Attachment menu: Add section for Videos

#### General information

* **App version:** 2.6.0 (122) , F-Droid
* **System:** OnePlus 5t, Android 9

#### Description

At the moment, Markor allows to take a photo using the system camera app, place it into the directory of the note file, and insert the relevant markdown into the file.

I would like to ask that videos (obtained form exactly the same source) be attachable exactly the same way.

If fact, if you use OpenCamera as your camera app, you can already _record_ videos from the summoned camera app, but those are rejected at the attachment time, and until you take an actual photo, the camera app wouldn't close.

If you use the stock one, however, it only allows you to take photos.

May I ask that such a feature be added? I think, this should not be a huge development effort.
(it's either that the app should not reject videos from OpenCamera, or just one button to obtain an image from an intent needs to be added, almost like the image acquisition one.)

#### Log

The program is not crashing.

Start by tracing Markor's existing photo attachment flow and the camera intent handling described in the issue; no files or tests are named. Done means videos from the camera source are accepted, copied beside the note, and represented by the appropriate Markdown attachment, without requiring a separate photo.

Written by the indexing model from the issue text.