Add separate keybind for selecting which gun ammo is used by default

[olanti-p]

Summary

SUMMARY: Interface "Add separate keybind for selecting which gun ammo is used by default"

Purpose of change

Fixes #1352
When pressing F to change gun mode if the gun has only 1 mode, and it's either RELOAD_AND_SHOOT (slings, bows) or RELOAD_ONE (shotguns, revolvers), the game will instead try to select default ammo to use.

There are a couple problems with this:

1. If none of the loading options are available (e.g. shotgun is loaded with regular shot, but you only have dragon breath left), the game will show a erroneous "You don't have any X to reload your Y" message.
2. If the shotgun has a second magazine (e.g. Kel-Tec KSG), it has a second firing mode, so the player loses ability to select default ammo.

Describe the solution

1. Add a separate keybind for changing default gun ammo, and remove default ammo selection fallback from F key.
   Now, if the player tries to switch firing mode, and the gun has only 1 mode, it'll say so.
2. Make "select default ammo" menu ignore current state of the gun and allow any compatible ammo to be marked as default.

Describe alternatives you've considered

Not adding yet another keybind, and instead do either of:

1. Restrict default ammo feature to RELOAD_AND_SHOOT guns, as they are the main use case.
   • Might be inconvenient when (or if) weapons start supporting mixed ammo as the player would need to re-select which shot to load each time they want to load one.
   • Might be inconvenient for people that rely on this feature to skip ammo selection when loading in first shot.
2. Don't change firing mode for RELOAD_ONE guns and instead go straight to default ammo selection menu.
   • Will make it impossible to switch firing mode for these guns outside target selection ui, which would be inconsistent with many other guns (rifles, pistols).
   • Will need to allow entering target selection ui with currently selected gun mode being empty (it's disallowed right now).

Testing

Marked some ammo as default using the new key or from target ui, tried reloading various shotguns and firing bows.

[AngelicosPhosphoros]

If possible, it would be good to make const item_location&.

[olanti-p]

Huh, looks like it was non-const to allow move-initialization of item_location for item::reload_option a few lines down, except item::reload_option's constructor takes a const item_location&, so the move does nothing.
Removed that move and changed ammo here to const item_location&.

[AngelicosPhosphoros]

const?

[Coolthulhu]

Consting primitives is a bit of an overkill, except where it really needs to be clear that this variable won't change.
Ideally, all primitives would be treated as const, except in cases where they obviously have to change (iteration, accumulator variable etc.).