Update some Doxygen settings

Enabled source code browsing and excluded enet from the docs by only
including the 'src' directory.
Removed deprecated and unused Storage::flushSkill
Small documentation updates for Point and Rectangle

This documentation used to be accurate, until Point was merged with the
Position class in commit 8ff3e6674c1d4fc05fc1ba87f42484689fca0879.

Also fixed a typo.
Used a typedef to make callback syntax more readable
Small simplification in Monster::died
Fixed setting current map for npc update function
Fixed and silenced compiler warning

Also removed a senseless line