Fixes Power Clip and Power Mask LPE issues about:

1. On undo/redo.
2. Convert to paths broken.
3. Power Mask not render well on reopen/other programs.

1. Some operations are missing on undo/redo previously getting some crashes and wrong on the undo stack. To fix it I remove the undo insensitive code, yes, now two undos for one UI operation... And add document done to be sure all steps are added to the undo stack. (previosly we miss last one of the verb)

2. When convert to paths we have wrong results like the effect is removed when visualy user spect keep. To fix I add to onremove function code to stop the normal process when keep paths is defined.

3. When reopening files with Power Mask with background set, in Inkscape, or GIMP for example, the mask is hidden. To fix I change the order of the added backround to first position as is the logial posiotion to be set (deep first).

Finaly I change a comparsion check wrong on remove LPE from verbs and add LPE remove finction to release clip/mask context menu actions.

Fixes issue #1144 (closed) Fixes issue #1162 (closed) Fixes issue #1158 (closed) Fixes issue #1101 (closed) Fixes issue #1039 (closed)

(cherry picked from commit caaa8086)