POSNK Panel widget interface

In order to implement a new widget for the "tardis" panel one has to provide at
least:
 * An empty paint method to override the WTK default paint-background method
 * A create-widget fucntion that sets w_mode and instantiates a WTK widget with
   a correctly set width field.

An example widget implementation is provided below:

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#include <stdlib.h>

#include <cairo.h>

#include <wtk/widget.h>

#include "tardis.h"

/* Paint method, called whenever the widget needs to be updated */
void example_paint(wtk_widget_t *w, cairo_t *context, int focused)
{
}

/* Process method, called periodically by the panel application, use this for *
 * dynamic content */
void example_process(panel_widget_t *w)
{
}

/* Creation method */
panel_widget_t *example_create()
{
    /* Specify the width of the widget, the other fields are set by the *
     * panel layout routine */
    clara_rect_t rect = {0, 0, 200, 0};

    /* Allocate memory for the widget object */
    panel_widget_t *w = malloc(sizeof(panel_widget_t));

    /* This is a fixed width widget */
    w->w_mode = PANEL_W_MODE_FIXED;

    /* Instantiate the WTK widget */
    w->widget = wtk_create_widget(rect);

    /* Set the callbacks */
    w->widget->callbacks.paint = &example_paint;
    w->process = &example_process;

    /* Optionally, set the implementation specific data pointer */
    w->impl = NULL;
    return w;
}

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