Multiple improvements for the scenario import panel

[mrvisscher]

Started out as a bugfix, ended up as a series of quality of life improvements for the scenarios import panel.

• Changed the scenario import toolbar to look a bit cleaner and to be more intuitive
• Added some statistics on the bottom so the user can perform some sanity checks on the scenario imports
• Adding and removing tables is now quicker if the tables are empty
• You can now actually switch the combination type after you've imported the files
• added by @marc-vdm: Resolve bug where duplicate flows would crash AB + improve excel/csv scenario exports

Demo

Related issues

• Fixes scenario calculation bug #1177
• Fixes Combine/extend options for loading several SDFs #1084
• Fixes Unnecessary SDF checks before file is loaded #1086
• Fixes LCA results display only the scenarios from the first scenario file #1143

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Link this PR to related issues by using closing keywords.
• Request a review from another developer.

[coveralls]

coverage: 50.927% (+0.05%) from 50.879%
when pulling 611f30f on mrvisscher:scenario-deletion-fix
into 6931bad on LCA-ActivityBrowser:master.