ShapeCraft 0.4.3-alpha

Fixed (found by updating real installs on all three systems):

• Two copies of ShapeCraft on one computer (an installed one and a portable one) no longer share their update folder,
  so one copy's update can't clear away the other's download.
• When an update can't be installed, ShapeCraft says why the next time it starts and tries again; with Update and
  Reopen it still reopens, on the version you had.

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Alpha: keep copies of anything important. ShapeCraft updates itself from here on: it downloads new
versions in the background and installs them when you close it.

First launch: macOS asks once: System Settings ▸ Privacy & Security ▸ Open Anyway.
Windows SmartScreen: More info ▸ Run anyway. Linux: make the AppImage executable, then double-click it.

Check a download against SHA256SUMS.txt: shasum -a 256 FILE (macOS, Linux) or Get-FileHash FILE (Windows).