Sort blocks in Frame Picker by index; allow search, order by name, moving

[nickberckley]

Because now blocks are directly stored on objects as references keymesh.blocks collection can be used in Frame Pickers UI list instead of filtered meshes (that means filtering can be completely removed). Because keymesh.blocks has internal index it means blocks in UI list can be ordered by that index instead of name, as they were before.

• But ordering by name is still desired, so this PR also adds name property to KeymeshBlocks PropertyGroup. Now UI list is displaying that name property, instead of the mesh name directly. The downside is that it needs update_block_name function on an update to make mesh names match KeymeshBlocks name, but the upside is that since UI list can optionally sort by name, and allows searching by it too.

• Since ordering is done by index now, we can also have operator that changes that index to reorder items in Frame Picker UI list. Two buttons are placed in the UI, similar to built-in UI lists in Blender.

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Other changes in this PR:

• block_active_index is moved from scene to object properties, where it should be
• "Purge Unused Keymesh Blocks" operator is moved up above move arrows
• When new blocks are added to the registry name property is set based on blocks name
• Little bit of formatting

[nickberckley]

Potentially, the fact that the block name is decoupled from the name of the ID it references could be an advantage because it allows to keep object data names frozen, while changing block names for the UI. I have mixed feelings about this, but might revisit.