Rename MapInfo::encounter_steps to MapInfo::encounter_rate

[fmatthew5876]

Leaving this one as a note to remember to do this later when we get the backlog of PR's merged in.

[Zegeri]

encounter_rate could mislead people into thinking that it's a ratio. For example, some people could understand encounter_rate = 10 as "10% chance of an encounter". encounter_steps leads to the correct interpretation, in my opinion.

[fmatthew5876]

I like encounter_steps, but then we need a different name for SavePartyLocation::encounter_steps.

We currently have these:

1. MapInfo::encounter_steps
2. SaveMapInfo::encounter_rate
3. Terrain::encounter_rate
4. SavePartyLocation::encounter_steps

(1) and (2) need the same name. I think (3) is appropriate because it's really a multiplier. (4) needs to be differentiated from (1) and (2). It's the number of steps at run time as the encounter algorithm runs.

[fmatthew5876]

We also have Game_Map::SetEncounterRate() and Game_Map::GetEncounterRate().

[Ghabry]

will add this one quickly before 0.8. Must have forgotten this one while renaming the other chunks.