Rollup of fixes for bugs found in 0.98

Fixes the following problems:
- On cold session boot with environment information in .RData, global environment is initially empty
- Checkboxes may be shown in locked environments
- Column widths may be wrong when repeatedly toggling between list and grid view and switching viewed environments
- Error management doesn't respect user code in R < 2.14 (disable and/or hide feature) 
- Breakpoints can't be set in R < 2.14 (show warning bar instead)