Removed ParameterValueWidget._addPopupMenu and _popupMenuDefinition, replacing them with PlugValueWidget._addPopupMenu and _popupMenuDefinition. This makes the parameter preset menus automatically available on all parameter uis, and provides for easy extension in the future (menus for manipulating plug inputs, expressions, locking etc). Note that if you previously had a custom ParameterValueWidget class with a _popupMenuDefinition() override, then it will no longer work - use ParameterValueWidget.popupMenuSignal() instead.
Removed ToolParameterValueWidget, replacing it with ToolPlugValueWidget which has identical functionality (but applicable to Plugs too).
Fixed bug which caused the filename column to become too wide when switching to Op browsing mode in the browser and then back to files.