Refactor/remove scenario.h and how the controls are managed #195
Description
The current implementation is unnecessarily redundant in scenario.h. One vector for all fields would be enough, which are by default hidden. Then in SetMode, they can be enabled depending on the mode.
Or even more improved, there is a mode object which does that.
There also needs to be an abstraction over the raw indexes of the fields, currently FID_SMALL_TEXT etc, because at the start, the context asks for how many fields there are. If we were to have a mode where there is a different number, the raw indexes do not match. This is required for #184 to work properly.