due to suspected instability with persistance Worker (randomly not persisted scenes, very hard to reproduce), switched to 'immediateSave mode' (at least for now)

This reverts commit 8fec7b23.

fixed a bug where erasing a pen stroke with simplifyPenStrokes=true ereased every overlapping polygons of it

fixed a crash where a scene was removed : the compactCache method has no more sense now that we load each scene to create a view on it (thumbnail views)

the last active page

This fixes a bug introduced in commit 8365f2f7 and improves the previous,
suboptimal behaviour. Now, polygon fill rules should be saved and loaded
correctly; and the changes should be reverse compatible (so that files
created with an older version will be loaded correctly as well).

(finished) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups

(WIP) fixed some bugs where 'Locked' and 'Visible on extended screen' properties were not persisted for groups'