All decoration, particle and module interfaces now takes a
pixman_image_t parameter, and all drawing is done using pixman APIs.
The wayland/xcb backends implement a new interface functions,
get_pixman_image(), that should return a pixman image instance that is
suitable for rendering.
In the wayland backend, the image uses the same backing data as the
cairo surface.
In the XCB backend, we create a new image each time, and then blit it
to the cairo surface at commit time.
In cases where it makes sense, use calloc() instead of malloc():
* When allocating large objects with many members, many for which
NULL/0 is a good default value.
* Arrays etc where we explicitly initialize to NULL anyway.
Since this struct only contained function pointers, make all particles
export those functions directly.
The plugin manager now defines a particle interface struct, and fills
it it by dlsym:ing the functions that used to be in particle_info.
If we have our own on_click handler, call the default
implementation. Otherwise, check which sub-particle the mouse is over,
and delegate the on_mouse() call.
This is intended to be a format-like string, with the possibility to
use tag formatters.
The expanded string will later be passed to the system() call.
This was always done before. Now that it is optional, one can for
example generate lists dynamically, using the same set of base
particles over and over again.
