To clarify term still frame: This is portion of strip that displays
static image. This area can exist before or after strip movie content.

Still frames were implemented as strip property, but this was never
displayed in panel. Only way to set still frames was to drag strip
handle with mouse or using python API. This would set either
seq->*still or seq->*ofs where * stands for start or end.

When strip had offset, it can't have still frames and vice versa, but
this had to be enforced in RNA functions and everywhere in code where
these fields are set directly. Strip can not have negative offset or
negative number of still frames.

This is not very practical approach and still frames can be simply
implemented as applying negative offset. Merging these offsets would
simplify offset calculations for example in D14962 and could make it
easier to also deprecate usage seq->*disp and necessity to call
update functions to recalculate strip boundaries.

For users only functional change is ability to set negative strip offset
using property in side panel.