Window-resize support for liquid_fb

Related to #740.
liquid_fb: dynamic reconfiguration support

With this patch, 'liquid_framebuffer' can be reconfigured at runtime.
The configuration arguments are now provided as XML attributes, matching
those for 'nit_fb'. Furthermore, two new configuration options are added:

<config ...
  resize_handle="off" - show/hide a resize handle widget in the lower
                        right window corner
  decoration="on"     - show/hide window decoration
                        (title bar and borders)
/>

Fixes #740
Fixes #14