Merge branch 'cache_for_particles' into 'master'

Faster!

This merge request consists of several independent commits which aim at making ToME4 faster:
1. Add cache for particles & shaders to avoid loadfile & loadstring in game;
2. Adjust the record of recent logs, making it faster and takes less space under extreme situation; 
3. Add cache for ShowChatLog dialog, making the scrolling smoother;
4. Reduce the frequency of player's 'hit_warning' particle effects, which might spam too much (each damage number may create a particle effect).

See merge request !749