Renamed GDML Worbench icon from "GDML_worbench.svg" to "GDML_worbench_icon.svg" because while building FreeCAD the compiler requires "GDML_worbench_icon.svg"
Manually created this .svg file. This PR also fixes a previous commit typos for AirplaneDesign and A2plus workbenches that fail to make them display correctly in the AddonManager GUI
Followup to PR #2369
* made appropriate addons/wbs lowercase
* used the correct case-sensitive spelling and formatting for addons/wbs
* updated .qrc file
* removed glider wb icon and mention in .qrc