[FEATURE] sap.m.Button: add ARIA/screenreader support

Added the ariaDescribedBy and ariaLabelledBy associations to the
sap.m.Button control. Also updated the explored page for buttons to use
the new associations. I added a new disabled button to the explored
sample to ensure that bringing over the code from the desktop version of
Button, which used a slightly different handling of the disabled
property, still worked correctly.

Fixed indentation and other whitespace

Standardized all JSDoc to use @return instead of @returns

Fixes https://github.com/SAP/openui5/issues/207

Change-Id: I57b1dfe9ad79eccd0d1f8dd0a8e2e2f194053db5
Merge "[FEATURE] sap.m.Button: add ARIA/screenreader support"