Snapping: Moving blocks in relation to other blocks

[swissspidy]

See #2875 for details.

Example: when moving a block, it should be easy to align it on the same X or Y position as another block.

Changelog entry

• Snapping: Added guidelines when moving blocks in relation to other blocks

[swissspidy]

See #2992 for screencast.

[swissspidy]

Testing Instructions

1. Create new story
2. Add a new block (so there should be 2 now)
3. Add background colors to both blocks
4. Move one of the blocks around on the page
5. For each of the block's sides (left, right, top, bottom) and centers (vertical and horizontal), verify that red snapping guidelines are shown in the following scenarios:
   1. When moving the block so it aligns with either side of other block
   2. When moving the block so it aligns with the center (vertical and horizontal) of the other block.
6. Verify that this also happens when there are N blocks on a page.

Basically, snapping guidelines should be shown to allow easily aligning blocks to the sides and center of the other. No actual snapping is done.

Next, verify that no guidelines are shown when dragging and at the same time holding down the alt/option key.

[csossi]

Verified in QA