Codechange: Move dropdown and slider out of widgets directory.

[PeterN]

Motivation / Problem

Source and headers for dropdowns and sliders are stored in the src/widgets/ directory, however all other widget code is in the main src/ directory.

src/widgets/ is otherwise only used for WidgetID enums.

These two uses are quite distinct and that they use the same subdirectory is unintuitive.

Description

Move dropdown and slider files out of src/widgets/ into src/. This leaves the widgets directory solely for defining Widget IDs.

Also shuffle some headers to place widget includes near end.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker