Add indeterminate style to checkboxes

[Henni]

The new checkbox style should support the indeterminate property.
Icons already exist: https://github.com/owncloud/core/blob/master/core/img/actions/checkbox-mixed.png

[MorrisJobke]

cc @owncloud/designers

[jancborchardt]

Yup. That’s why I added it there. Mostly this will be useful for Files multiselection, and an indicator that only some items are selected.

Also related to #19564

[jancborchardt]

@Henni the icons were added some time ago, didn’t you also add the style for it?

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.