Fix some issues with the AppImage launcher script

[Reflexe]

Fixes #5284

[Reflexe]

I will wait for circleci to pass and then rebase it for stable.

[LmmsBot]

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Linux

• Linux (AppImage): lmms-1.2.0-rc6.753-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.0-rc6.753-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.0-rc6.753-mingw-win64.exe (build link)

macOS

• macOS : lmms-1.2.0-rc6.753-mac10.13.dmg (build link)

🤖

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[Reflexe]

Tested it, working great.

> mkdir "Directory With Spaces"
> mv ~/Downloads/lmms-1.2.0-rc6.753-linux-x86_64.AppImage Directory\ With\ Spaces/
> Directory\ With\ Spaces/lmms-1.2.0-rc6.753-linux-x86_64.AppImage

> cd Directory\ With\ Spaces
> ./lmms-1.2.0-rc6.753-linux-x86_64.AppImage

> ./lmms-1.2.0-rc6.753-linux-x86_64.AppImage --appimage-extract
> cd squashfs-root 
> ./AppRun