New Database terms for Maniacs

[florianessl]

A few new term fields have been added in the latest version of Maniacs, to provide more customization options for some in-game texts:

• Experience gained
• Level Up
• Items received
• Skill learned

I would have preferred some template strings similar to the ones found in RPG2KE, but they do the job..

I moved those message texts to "battle_commands" which I then renamed to "game_message_terms". The terms code is all over the place & a lot of refactoring should be done here to make localization less of a hassle.
I'm sure this issue came up before & we could use this PR to do also some housekeeping here. 🤔

Maybe move everything related to in-game terms to its own naming scheme? (terms_battle.cpp, terms_actor.cpp, etc.)

New custom fields for liblcf:

Terms,maniac_item_received_a,f,DBString,0xA1,,0,1,
Terms,maniac_level_up_a,f,DBString,0xA2,,0,1,
Terms,maniac_level_up_b,f,DBString,0xA3,,0,1,
Terms,maniac_level_up_c,f,DBString,0xA4,,0,1,
Terms,maniac_exp_received_a,f,DBString,0xA5,,0,1,
Terms,maniac_skill_learned_a,f,DBString,0xA6,,0,1,

[florianessl]

Oops, it wasn't my intention to push this version, which already merges in some stuff not yet in master. I'll rebase & force-push that later.

Another thing I noticed:
Feature::HasPlaceholders() really only affects messages of the battle scene. Makes sense, because the associated database field is called easyrpg_**battle**_use_rpg2ke_strings, but the way it is described in code doesn't make it immediately obvious.
I stumbled upon this after finally migrating VDCE to 2k3 & then had these broken placeholder Level-Up messages. I had to change it back to use the non-template way of stitching together strings. :/
The level-up messages are actually used inside & outside of the battle, so this is also a bit inconsistent. But just renaming the flag & make it affect all types of placeholder messages could maybe break the text of some games.