Feature: Remember the last-used signal between games

[2TallTyler]

Motivation / Problem

When the player opens the signal GUI, they find that last signal they used is selected. However, when starting a new game, this is not remembered and the selection defaults to a one-way path signal.

Before #8688, there was a setting to choose this new-game default, and the removal of the setting has generated many complaints.

Description

The last-used signal type is saved to the config file and initialized as the default when starting a new game.

It is also the selected signal if the signal GUI button is Ctrl-clicked on to select the signal tool without opening the signal GUI — reverting to pre-#8688 behavior.

Closes #9705.
Closes #9631.

Limitations

When a non-path signal is selected and then the signal GUI is reduced to show only path signals, the selected non-path signal remains the default and you can build them until you select something else. This is behavior introduced in #9617, but now it persists between games.

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, gs_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