Libre Panel v0.5.1

Fixes from the first installs of 0.5.0 on Windows, above all the app's window.

Fixes

• Windows: the app's window did not open. It died at once, before it showed, on every PC: it was
  given a PNG as its icon, which .NET cannot use as one. It now takes the program's own icon. A
  window that still cannot start (a broken WebView2) opens the app in the browser instead of
  nothing, and logs/window.log says why. The release build opens a real window on Windows
  before it is published (libre-panel window --check --open)
• A theme that config.toml names but that is not installed any more (removed, or one of a plugin
  that is gone) no longer leaves the panel dark until the file is changed: the panel shows its
  own theme (Lumen on the 8.8" and 9.2", Libre Default elsewhere), and the tray menu, its tooltip
  and Start name the missing theme and say how to choose another. config.toml stays as it is
• Windows autostart: a task named "Libre Panel" in Task Scheduler that is switched off, or that
  starts another program (a leftover of an older copy or of a mod), showed Start with Windows as
  on and kept the normal start from being written. Only Libre Panel's own task, switched on,
  counts now; another one is left alone, and Settings → System and libre-panel autostart status say how to remove it

Which file? Windows: …-setup.exe, Linux: ….AppImage; see Install.