feat(form): add touch screen support for dnd

[miguelgrc]

Closes #50

Swapped react-dnd-html5-backend with react-dnd-multi-backend. The alternatives proposed in #50 are not ideal:

• Adding the option for users to provide their desired dnd backend is not that simple, since they would need to take care of choosing one or the other depending on the environment, which would usually vary.
• The alternative of waiting for Make dnd optional #17 to be merged and then disabling it by default for mobile, removing the select field column and having only two columns (the schema tree and the schema preview which would also be used for the property editor) is not ideal as the screen space would be a bit too narrow to have those two columns side by side (it is better to have the schema preview as a full-width column below).

The chosen solution also has its drawbacks (a slightly bigger bundle size) but it's the simplest and maintains the dnd functionality on all devices, which is the most comfortable way of using formule.

Something else that could be done in this direction is to have the dnd dependencies as optional peer dependencies and let users install them only if they want to use the dnd, but this would be trickier to set up.

[pamfilos]

remove/replace also in other components. e.g.: src/admin/components/SchemaWizard.jsx