Fix bugs related to Trainer Slide messages

[DizzyEggg]

Fixes #2693
Fixes #1655

Note:
The issue Lunos submitted covers many topics. In this PR I only fixed graphical glitches and the feature not functioning properly in doubles.
Once it's merged, I believe it would be best to have two new issues:

• the first one for the trainer sprite appearing on the mon sprite in doubles, not glitchy, but looks weird, could be improved
• making the slide messages appear at the same moments as in the games, that is adding a super effective field and changing 25% msg to 50%(or having both, that's subject to discussion and change)