Update VE core submodule to master (15e5545)

Authored by Jdforrester-WMF.


Update VE core submodule to master (15e5545)

New changes:
87c538a PasteRules#plainText: Fallback if rules.external is undefined
7155191 Localisation updates from https://translatewiki.net.
c2d03ea ToolContextItem: Use the target's command registry
d196838 DataTransferHandler: Improve aborting and resolution/rejection
009190e Allow the target to specify a DataTransferHandler factory
0ea9439 Split out basic functionality of tableCell so other nodes can behave as cells
999a5da ve.ce.TableCellableNode: Only apply title to nodes which are cellable

Change-Id: I56a73a26f21a6e22b6bafc850cdee35397e5d15d