To make Vala happy, the invoker and virtual functions have to have to same prototype.
Here for GESTimelineElement, a possible approach would be to add a `gboolean` as return type of the invokers

The virtual methods:
```
  gboolean (*set_start)        (GESTimelineElement * self, GstClockTime start);
  gboolean (*set_inpoint)      (GESTimelineElement * self, GstClockTime inpoint);
  gboolean (*set_duration)     (GESTimelineElement * self, GstClockTime duration);
  gboolean (*set_max_duration) (GESTimelineElement * self, GstClockTime maxduration);
  gboolean (*set_priority)     (GESTimelineElement * self, guint32 priority);
```
The invokers:
```
GES_API
void ges_timeline_element_set_start                  (GESTimelineElement *self, GstClockTime start);
GES_API
void ges_timeline_element_set_inpoint                (GESTimelineElement *self, GstClockTime inpoint);
GES_API
void ges_timeline_element_set_duration               (GESTimelineElement *self, GstClockTime duration);
GES_API
void ges_timeline_element_set_max_duration           (GESTimelineElement *self, GstClockTime maxduration);
GES_API
void ges_timeline_element_set_priority               (GESTimelineElement *self, guint32 priority);
```