Restack extension: add options to reverse, shuffle Z-order of selection

[su_v]

Attached diff adds a new option 'Reverse' to the 'Arrange > Restack' extension.

The 'Reverse' option acts in two modes, depending on selection:
1) if selection is a single group, reverse Z-order of objects inside group
2) if selection has more than one object, reverse Z-order of selected objects

The diff was created with minimal changes in mind for easier review - the indentation in the new method restack_spacial() would be adjusted in a second commit).

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Further refactoring (whitespace, PEP8) possible (not strictly required), see
http://bazaar.launchpad.net/~suv-lp/+junk/inkscape-extensions/files/head:/arrange/

[su_v (suv-lp)]

Second version of the patch fixes translation support (add 'inkex.localize()' in the script; update po/POTFILES.in).

[su_v (suv-lp)]

Third version of the patch adds another new option 'Shuffle' to the 'Arrange > Restack' extension (based on user feedback about the added 'Reverse' option).

Like 'Reverse', 'Shuffle' can restack objects inside a single selected group, or a selection of multiple objects.

[su_v (suv-lp)]

Final version of the patch for tonight: support arranging objects within a single selected group for all options (for consistency reasons).

[su_v (suv-lp)]

Patch v5 includes refactoring of the GUI in an attempt to group related options with notebook tabs as best as possible:

Tab 'Positional': original options
The 'Restack Direction' option was converted into an inline notebook with 2 tabs: Presets, Custom (arbitrary angle). Only the active (visible) tab is used for Live Preview and Apply.
Below the direction notebook are the remaining options (reference point of the objects' bbox) which affect all directional restacking.

Tab 'Stack': new options 'Reverse' and 'Shuffle'
This tab has restack options based on the original z-order, without taking the objects' position on canvas into account.

Tab 'Help': help text

[su_v (suv-lp)]

Committed in rev 14424.