Merge branch 'release/3.0.0' into develop
Bumped the develop branch to version 3.1.beta1
Merge branch 'release/3.0.0' into develop
Merge branch 'release/3.0.0' into develop

Conflicts:
	system/config/constants.php
The pageTree widget is now sortable just like the fileTree widget and the custom and quick navigation modules consider the custom order (see #4936)
Do not show the page/file tree drag hint if there are no files yet
Merge branch 'release/3.0.0' into develop

Conflicts:
	system/config/constants.php
	system/docs/CHANGELOG.md
	system/modules/core/widgets/PageTree.php
Language dialects such as `fr_FR` are now supported (see #4836)
Language codes now differ from locales (see #4836)
Updated the language config file (see #4836)
Updated the English source translation of the language names
Added `it_CH` (Italian Switzerland)
Fixed a small typo and added the German translation
Fixed a few minor issues
Added the classes "toggle_desktop" and "toggle_mobile" to the desktop/mobile switch link (see #4975)
Moved all icons shared in the back end and front end to `assets/contao/images`, so there are no more interrelations between the front end and the back end theme (see #4662)
Moved `Controller::getTheme()` to `Backend::getTheme()` (see #4662)
Moved `Controller::getBackendThemes()` to `Backend::getThemes()` (see #4694)
Moved `System::splitFriendlyName()` to `String::splitFriendlyEmail()` and made the `Email` class independent from the `System` class (see #4694)
Moved `Controller::generateImage()` to `Image::getHtml()` (see #4664)