kiosk-shell: add zorder support

Add the capability for specifying the relative stacking order of surfaces. This is accomplished by imbuing a meaning to the position within the "app-ids" list in which an application's name is mentioned. Apps listed earlier in the list are stacked below apps listed later in the list.

For example,

[output]
name=screen0
app-ids=org.domain.app1,com.domain.app2

would cause app2 to be stacked above app1.

Applications not registered in any particular output's app-ids list will continue to be stacked at the top of the view list as it exists at the moment the surface first commits a buffer.

Signed-off-by: Matt Hoosier matt.hoosier@garmin.com