Restore "hide-upload-button" option from 0.17
Restore "hide-upload-button" option from 0.17

When using the upload component within a larger form, it may be desirable to disallow file uploading directly from the component. 0.17 had this option. Currently 1.0-beta requires the entire header to be overwritten via scoped slot to hide the `upload` button.

This update also hides the `add` button when the `multiple` option is not enabled and a file has already been selected.
pdanpdan/quasar,fsgiudice/quasar,pdanpdan/quasar,fsgiudice/quasar,rstoenescu/quasar-framework,quasarframework/quasar,pdanpdan/quasar,rstoenescu/quasar-framework,pdanpdan/quasar,quasarframework/quasar,rstoenescu/quasar-framework,quasarframework/quasar,fsgiudice/quasar,quasarframework/quasar