Fix display of some math symbols

Fix display of some math symbols

* Fix spacing in lib/symbols after recent commits about math spacing, as well as
  older spacing issues (e.g. \Join).

* InsetMathKern now uses the same em value as other math length commands.

What is nice is that the kerning amount now matches the ones found in the
packages definition (modulo 10mu that lyx currently adds between relations).

Testcase: $\CheckedBox\LEFTcircle\RIGHTcircle\photon\gluon\vcentcolon\dblcolon\Coloneqq\eqcolon\models\hookrightarrow\bowtie\hookleftarrow\Join\APLinv\neq$