Change: always allow to click on the toolbar icons for road/rail/dock/airport

This change allows a user to see what is available and what will become
available before it is available, instead of only disabling the button
with no further explanation. It also always allows building roads and
canals, even if no vehicles are available for road / water.

For rail/road/tram, a dropdown with available types is shown. If
none are available, it reads "None". If the type is not yet available,
it is greyed out.

For dock/airport, this always open the toolbar, but building airports,
docks, and depots buttons are disabled till vehicles are available
for those.

Road is the only exception, with the primary road always being
available. Here too, stations and depots are disabled till vehicles
become available. It does mean you can now always build roads to
for example help towns grow.

Codechange: Switch DropDownList to directly use std::vector, thus making AutoDeleteSmallVector obsolete.

DropDownListItem are strongly managed using std::unique_ptr to ensure leak-free handling. Appropriate use
of move-semantics make intent a lot clearer than parameter comments and allows the compiler to generate
copy-free code for most situations.

Change: Shorten engine rail type drop down in autoreplace window. (#7448)

In the autoreplace window, the rail type drop down is for choosing engines
of the given time. Many rail types do not have engines specifically designed for them,
and are merely compatible with other rail types. This list is thus unwieldy and many
options have no engines available.

As this drop down is for choosing _engine_ rail type rather than compatible rail types,
we can list just the rail types explicitly listed by engines.

Codechange: Change element type used for rail type usage stats array in SetDefaultRailGui

The array is rail type sized in terms of number of elements.
Each element should be a unsigned integer, not a rail type itself.
This fixes runtime warnings reported by UndefinedBehaviorSanitizer.