MNT/BUG: Misc fixes

[tangkong]

Description

• RangeWidget's visualizations update a bit more frequently, and also the label text actually updates. Closes GUI: Range Comparison number line does not update with changed values #212
• Adds a menu option to open the welcome tab, since people like it. Closes Instead of "new file" dialog show "atef welcome" #201
• Properly shows an error message box when a file can't be opened. Closes BUG: atef still attempts to open a file if it cannot be loaded #202
• Removes child AtefItem from a ProcedureStep when it's changed from the specific-step-combobox. Closes BUG: Switching the step type may leave behind children tree items #195
• Allow tolerances to be None in Equals comparison. Modifies the line-edit setup to allow null values ('', None) when casting the line edit value. Closes BUG: Problematic Relative Tolerance in ATEF Comparison Settings  #128

Motivation and Context

An assortment of fixes because I didn't want to work on anything big but still wanted to be a bit productive

How Has This Been Tested?

interactively

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Code has been checked for threading issues (no blocking tasks in GUI thread)
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[ZLLentz]

I think all these fixes look correct to me