starmath: remove unused resource ids

Change-Id: I9dee3cf5591df19cd0189e32ae7be024b07020f3
fdo#71580: Elements dock: Alignment items are missing

Also add << and >> to the Elements dock.

Change-Id: Id58d64c8ab938259e84ce1a178644c6d4a6ee95d
starmath: some improvements

Change-Id: I9dbeb16a527094bb8b32f8b8926f2a511e6190ba
starmath: use global resource string

In favor of many getters and setters :)

Change-Id: Ibdfe6e2cb70ec2e743def2f898c1b56713af2661