Add support for re-requesting icons

[moertel]

This commit changes the default behaviour and adds new behaviour hidden behind a new boolean option enable_icon_request_multiple.

• When enable_icon_request_multiple=false (default), users cannot select icons they already requested but instead see an info dialog.

• When enable_icon_request_multiple=true, users see a confirmation dialog for icons they already requested, asking them whether they are sure they want to spend another request on them.

The old default behaviour was to let the user select already requested icons but then display an error dialog when they hit the "send" button. From a user perspective it never made sense to let them select it in the first place, as the result was 100% sure to error out.

The new default behaviour happens immediately on click of the checkbox.

The new multi-request behaviour behind the flag is useful for those who want to support a voting system for free requests (i.e. let users cast multiple votes for the same icon), and useful for those with both free and premium requests (e.g. when users want to "top-up" their free requests later on to speed them up).

[moertel]

enable_icon_request_multiple=false	enable_icon_request_multiple=true

[moertel]

FYI, this has been live in my apps for about a month and is working fine. 👍

[moertel]

@sarsamurmu I figure you're busy but I wanted to give this PR a ping as it's been running fine without any problems in my app since October. :)

[sarsamurmu]

Thank you for your contribution 😃