svx: fix SID_OFFER_IMPORT and SID_ATTR_CHAR_GRABBAG having the same SID

As commit 669ad519902e21e0cb3537fb7e203987f75a4077 (colliding
SID_SVX_START based ids, 2013-06-24) suggests, new SID_SVX_START-based
SID's shouldn't be added to include/editeng/editids.hrc. However, if it
was done, at least add a comment about it to include/svx/svxids.hrc, so
it becomes obvious that the two slots have the same ID.

This makes Format -> Frame -> Area -> Bitmap have the Import Graphic
button again.

Change-Id: I2fc852200d3cc5dfbf073ed615ac83a46040e16c