Layers dialog: Update to 2.99
- List additional buttons in the bottom bar under "Layer management".
- Use consistent capitalization among "Layer management" options.
- Cover new "Lock visibility" option.
- Update screenshot (lock row moved to column attribute with a new overlay for locks; additional button in bottom bar).
- Replace existing three Lock option screenshots by their corresponding icons only (to reduce maintenance costs); add icon for new "Lock visibility" option.
- "Chain layers" moved down to bottom bar and renamed to "Select layers by patterns".
- Remove outdated image of layer list entry (outdated and self-explanatory given all the text around).
- Update the two alpha lock example screenshots by removing their obsolete chrome to reduce maintenance costs (and double their size because it's hard to see anything otherwise).
- Correct duplicated layers name suffix.
- Link "merging layers functions" to its actual docs instead of unhelpful toplevel main menu page.