Lots of improvements, manual locale, Crostini on Chromebooks etc...

• There have been the usual minor bug fixes and clean ups from various regular contributors and some new ones too. Thank you for your careful and well targetted changes.
• Carlos (@carlosperate) fixed some packaging problems relating to the iPython kernel bundled with Mu.
• Martin (@dybber) fixed a couple of problems relating to the stopping of child processes (Flask and scripts stopped via KeyboardInterrupt in Linux).
• The web mode checks for the availability of templates in the local directory tree before starting up. If a template directory isn't found in the expected location, then the user sees a helpful message describing the problem and what they need to do to fix it.
• Mu's splash screen no longer always appears on top of everything else on the user's desktop. The splash screen now also logs the progress of installing the various packages needed on first install. Thanks to Carlos for these changes.
• A new admin/settings option has been added to allow users to manually change the translation Mu uses for its interface. Updating this setting requires a restart of Mu. Zander (@ZanderBrown) contributed the icon/glyph to indicate the relevant tab is for translation related settings (not entirely obvious if Mu's UI is using a language you don't understand and you're looking for the setting that relates to translations). The icon makes this clear.
• On some desktop windowing systems there is a bug that means windows re-open at a position higher up the screen, and so may appear off the top of the screen. We've ensured this never happens with Mu. If Mu starts with any part of the window off the screen, the window is moved to be within the dimensions of the screen. This was a weird one to track down and fix.
• Many thanks to Ethan Spoelstra (@espoelstra) who contributed a change so Crostini on ChromeOS is used to return the correct CIRCUITPY path if it exists.
• Huge thanks to Keith Packard (@keith-packard) for several contributions to this release of Mu. Keith refactored the way in which Mu handles pasting in the REPL window so it works correctly and more broadly across operating systems.
• Keith also fixed some font related issues in the REPL.
• Keith was on fire with a couple more contributions relating to SVG icons in the buttons in Mu. We're very grateful to Ben Williams (@Rybec) for putting in the work to make our button icons SVG files. Keith made the code changes to implement this.
• Thanks to Miro Hrončok (@hroncok) for pointing out a change in Python 10 which would break some of our UI calls into PyQt, and who provided a patch to fix things.
• Some minor clarifications in our developer documentation (https://mu.rtfd.io).