Update VE core submodule to master (f173ff2)

New changes:
0d7ee1a Resize handle images
83bbf97 doc: Use lowercase types where primitives (not objects) are expected
2808b15 Localisation updates from https://translatewiki.net.
010038c Update OOjs UI to v0.1.0-pre (68c91ae065)
76d7500 Localisation updates from https://translatewiki.net.
d787eb7 Add placeholders to MediaSizeWidget
baa64db Allow enforce(Min|Max)Dimensions to be changed
3afe645 Add 'change' event to MediaSizeWidget
0886f67 Localisation updates from https://translatewiki.net.

Change-Id: I5cb88b1cbf1b64de979fb393dcbb3e6b304766bc