P: Add collage/composite to LR C

There are already features that create new images in LR, so why not add collage, and perhaps compositing.

There are a whole bunch of ways in which such a UI could work, but ... here's something fairly easy to visualize in the case of collage.

• User has a panel from which to select collage layouts. Let's say for now they're all rectangular.
• User selects a layout, along with color/gradient/photo to use as background, etc., then selects photos to drop into it.
• With photos in the layout, drag to swap them, click and hit delete to remove, etc
• Resize with scroll wheel, also the usual "fit/fill" etc
• Move borders by grabbing the border and dragging
• Widen/narrow borders by grabbing border edge and dragging
• Draw new slots with right click-drag rectangle, or whatever
• Overlapping slots should be fine
• Delete slots in one of the obvious ways, right-click context menu, click-select delete key, whatever
• Border changes have optional snapping to make it easy to keep things aligned

Ideally the collage could be saved in a form that allows for future editing, updates when embedded photographs change, etc., but if not, hey, that's still 100x better than the current situation.

Note that this is pretty similar to how print and book modules do or should work although they are creaky as h-ll and in desperate need of a good update.

For composites, trickier, but the spiffy new masking tools would be awesome to have. Heck, just being able to create and export an alpha channel would be a start.