MDL-44670 themes: Updated screenshots of Clean and More
MDL-44590 category: correct get_string component
MDL-44590 behat: Add test for course category idnumber duplication
MDL-44739 theme_more: Set default theme settings
MDL-44804 editor_atto: JS error when filepicker is disabled
MDL-44822 assignfeedback_editpdf: Stop test generation in is_enabled()

This change also moves one of the checks in test_gs_path. If
generateimage is false, there's no need to stat for the fixture file.
MDL-44701 gradereport_grader: Display warning on navigation if unsaved grades are present
MDL-44837 Behat: 'dialogue' selector only works for confirm dialogues

The dialogue selector in Behat (used only in one place) didn't work for
other types of standard Moodle notification dialogue.
MDL-44733 add support for disabling of scheduled tasks
MDL-44715 use parent::validate_data() consistently in all events

Includes some minor PHPDocs typo fixes.
MDL-44754 Atto: Explaining what group names are used for in toolbars
MDL-44808 atto_table: Remove accessibility information from dialogue
MDL-44811 quiz settings form title: change for consistency.

Thanks to Fred for poining out that most other activities use the work
Appearance for these sorts of settings.
MDL-44746 core_events: removed incorrect uses of snapshots
MDL-44746 core_events: minor changes to the user_deleted event
Merge branch 'MDL-44804-master' of git://github.com/FMCorz/moodle
MDL-44767 atto_editor: Browser hangs when making a selection
MDL-44764 atto_image: Reword the alt related labels
MDL-44755 atto_editor: Remove hardcoded icons from settings

Previously, we hardcoded a list of icons for Atto plugins. This is not
a good idea as it would be very quickly outdated, or incomplete. Now,
we proceed as we often do: if the image pix/icon exists in the plugin
it is displayed, else nothing is displayed.
MDL-44747 editor_atto: Improved validation of toolbar config

This ensures that:
- Plugins are only used once,
- Group names are unique,
- Lines match: group = plugin[, plugin[, plugin ...]],
- There are some groups and plugins defined,
- The plugins used are installed.