It used to be a random sentinel for end-of-(widget-)list that was used to tell
that no action has taken place yet. Since the last action is practically the
widget that was pressed, add the sentinel to that enumeration.
This simplifies processing nwidget parts as, unlike the remaining length, the pointer to the end of the list never changes. This is the same principle as we use(d) for tracking end instead of length for C-style strings.
And this removes 160~ instances of the lengthof() macro.
Hotkeys are now initialized inline, and use std::vector instead of
separate static C-arrays and std::string instead of char *. The list end
marker is no longer required.
GetScrolled*FromWidget took line height from the widget's resize_y value,
however not all widgets are resizable, resulting in a division-by-zero.
Allow passing line height explicitly in cases where a widget is not
resizable.
In many instances the clicked row position is 'manually' calculated
instead of using the GetScrolledRowFromWidget helper function, with
variations on checks. Replace with the two helpers where possible.
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.
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.