adding basic commands and updating style
SWC-1223: fix spacing, styling of gxt dropdown, and add one of the free glyphicon pngs to the project
add strikethrough button
fix strikethrough icon
if the selection is already surrounded by the basic markdown, then undo the effect
on invalid selection, pop up error
move messages to displayconstants
from code review
Merge pull request #853 from jay-hodgson/SWC-1223

Swc-1223