Fix the anchor `class` attribute in the tree view

Single HTML element attributes must be separated by whitespace characters. In the tree view, that is currently not the case for the default button operation's `class` attribute, thus making the markup invalid. The present patch fixes the issue.

Signed-off-by: xchs <github@gmx.net>
Fix an unescaped ampersand in the `PageTree` widget (hopefully the last one)

Signed-off-by: xchs <github@gmx.net>
Merge branch 'hotfix/class-attribute' of git://github.com/xchs/contao-core into develop