ENH: plugin activate / deactivate

Some enhancements and fixes:

- Use a checkbox to enable/disable plugins. The method using right-click was not intuitive to me.
- Plugins are now always disabled when they are first loaded
- Update GUI immediately when enabling/disabling a plugin
- Fix a segfault that occurred when selecting inactive and active plugins while the measurement is running
- Prevent user from enabling/disabling plugins during the measurement