Port the changes from contao/core@db9aa04d4166eb9190a336535e796e77d2db2b9d
Adjust the Composer requirements
Add the minimum PHP version
Revert the Symfony related changes
Return `null` if a widget is empty and the DB field is nullable (see contao/contao#17)
Move the Contao `DiffRenderer` to the core package
Remove a leftover Symfony reference
Cache the installed package versions (see contao/contao#25)
Port the changes from contao/core@1dfb1649301a3e40723b781e59e02faeb8382402
Port the SVG changes from contao/core#7108

- contao/core@15201d0aa22ae777dd977bd368f49f849a64ee83
- contao/core@5858e97d73cf1174e6dcc867c28c0950524311a1
- contao/core@a2b95a2eb6a984377ff57154bdd4547cc69b7dd6
- contao/core@953d3617b1747d10dcb5f159fa69b101ba128fd5
Port the changes from contao/core@89cd48af21f88a1ca45428a5f197aee177bd61f9
Port the changes from contao/core@fe1f0e90ef1fadabe07c152e64fdae79d020e46f
Port the changes from contao/core@b20d4554e7e4deb12f6ba6efb136d176770d3ec7
Merge branch develop into feature/picture-element
Add resize SVG tests
Add test for Image::getPixelValue()