Jog step logic changes to support older 4.x Qt

Also added "Jog" text to 4th axis
Fixes to jog step persistence conversion
Fix the remaining compiler warnings on MacOS

Clang doesn't like the gcc pragmas to ignore warnings; since
this is used just to ignore unused variables, we can use the
Q_UNUSED() macro instead (which just expands to (void)name
anyway which is portable).
Merge pull request #69 from chrisy/feature/clang-warnings

Fix the remaining compiler warnings on MacOS/clang (From Chris)
Add grbl status icons to mainwindow

- added icon showing spindle state
- added coolant icon
- added tool number and feed rate icons
- refactored the incorrect measurement parser
- cosmetic formatting fixes

Add tool number and feed rate icons