Update VE core submodule to master (7004717)

New changes:
6d8a59f Create ImageNode mixin to share logic between Block & Inline
aa6765f Fix `ve.dm.Transaction.newFromReplacement` when `removeMetadata` is true
a6e7a0c Set expanded=false on command help dialog layout
029dbe4 Allow override of window manager in language input widget
8407325 Annotate inserted special characters
4666caf Localisation updates from https://translatewiki.net.
f67e81e Factor out ve.dm.Transaction.pushReplaceInternal
7004717 Prefer String#slice over substr() or substring()

Change-Id: I72dbf37750b5582049cb0fb2730a99066a64b067