The cancel button was overlaying the submit button preventing the submission of a prompt.
Distinguish between cancelable and mandatory prompt.
Only^ allow cancelable prompts to be removed.

Fixes: 37acd982

* implement events instead of interfaces
  Interfaces and the wrappers head the problem that they could not support multiple events at once or only a single one without loosing the others.
* add text input support for WASM on mobile
  A hidden html input is required to open the keyboard and receive the
  input.
* improve the number choice
  * fix using the default number
  * add submit button only if a onclick method was provided

* Generalize hover detection and move it into ui.Collection
* Implement transparencu for the prompt banner

* fix a lot of target and action bugs
* check target selection before adding it to the prompt
* use the store view for stores on the map
* fix card highlighting in CardGrid
* Only prompt for Upkeep-/DiscardActions if appropriate
* Add helper for areaEffects granting new FullActions
* Add Target() helper selecting the first target to the Action interface
* Fix BuyAction and DiscardAction targets
* Remember the stores a player has seen