BoostSet: Add 'remove' and rename BoostHash::erase to remove

This is more consistent with Qt's method naming.
Core/MRI: Fix handling of Etc/Font properties

The gist of it is that for Etc and Font props, the assignment
operator (eg. 'sprite.color=') does not take a reference of the
right hand parameter and replaces its previous one with it (this
was the old behavior). Rather, it keeps its internal property
object and copies the parameter object into it by value.
The getter is unchanged; it still returns a reference to the
internal property object.

s = Sprite.new
c = Color.new
s.color = c
p s.color == c # => true
p s.color.object_id == c.object_id # => false (true before)
c = s.color
p s.color.object_id == c.object_id # => true
Bindings: Remove 'wrapNilProperty'

It's completely useless lol.
Small compilation fix (mruby is still broken though)