Make included properties' names shorter

This fixes #786. Now included properties won't have `_fill_`, `_line_` or `_text_` infixes, e.g.:
```
In [1]: from bokeh.models import *

In [2]: p = Plot()

In [3]: p.outline_
p.outline_alpha        p.outline_color        p.outline_dash_offset  p.outline_width        
p.outline_cap          p.outline_dash         p.outline_join         

In [3]: p.outline_color
Out[3]: 'black'
```
Previously it was `p.outline_line_color`. This works, because currently included properties allow only one mixin. If this changes, the approach implemented in this PR will have to be refined.