Introduce type auto at many places

Replace the explicit types (mostly Rectangle) by auto in a lot of local
variable declarations.
Make Color, Rectangle, RectangleVec classes
Introduce namespace herbstluft

Put only the x11-types.h (Color, Rectangle, RectangleVec so far) into
that new namespace herbstluft.
Add new basic object code

Add basic classes (Entity, Attribute, Object) for the new object tree.
Make HSClient a class

Make all client_*() functions member functions of that HSClient class.
Introduce setter and getter for HSClient::tag_