Allow plugin variable names to include numbers

An example name would be 'extruder1'
Allow plugin variable names to include underscore

For instance 'big_cheese'
Merge remote-tracking branch 'upstream/master'
Fixed filename typo sliceProgessPanel.py
Fixed class name typo sliceProgessPanel
Fixed filename typo exporer.py
Fixed class/method name typo 'exporer'
Merge pull request #341 from smorloc/master

Permit numbers & underscore in plugin variable names