New LPE Slice

Add new LPE (live path effect) Slice.

This add a way to multi slice a SPLPEItem (shapes,paths and groups currently) alowing transforms and style changes in all normal benefits of LPE.

Also this branch extend the LPE system to know about previous and next effect of the current LPE. This is used highly on the MR, so only last slice process the final output, but we go throught different position in stacked continuous "slice" effects to make things like:

• Get the line separator from all consecutive slices
• Apply parameters in any LPE in the stack affecting to the top one
• Update on apply existing items to keep styles used
• Allow reset styles in any effect

Another feature is the system add another way to update the effect on do after effect passing the curve after apply to allow update it (previously you only can change groups in do after effect).

This LPE remove a GSoC proposal.

TODO/KNOW ISSUES:

• Add icon adam.bellis and datalot done it
• Find a way to keep styles on remove slices (Probably wont fix)
• Find copy/paste/duple/stamp multiple for LPE system.
• Order properly extra items (datalot).

Here are 3 gifs (could be slow):

Open Gif 1, PowerTux

Open Gif 2, Simple shape

Open Gif 3, Text clone