Add option for portable installation

[GJ]

Portable installations of mixxx are very convenient.
Windows 8.1 x64; i5 4th gen CPU.

[GJ (gj23)]

For what it's worth, the following workaround has been successful for me on x64 installs:
1). Install the portable version of Mixxx from https://portableapps.com/node/23093
2). Install the latest x64 beta of 2.1
3). Copy the contents of the beta folder into the portable folder, for example copy c:\program files\Mixxx to A:\MixxxPortable\App\Mixxx\

My point is that whatever existing code allows Mixxx 2.0 to run as portable seems to be sufficient to run 2.1 as portable.

[Sébastien BLAISOT (sblaisot)]

We have everything needed to launch Mixxx as a portable application, using command line parameters --resourcePath and --settingsPath.

My plan here is to add a portable launchscript for windows, Linux and MacOSX using these options to launch Mixxx using only "locale" resources (i.e. inside the current working directory) and update the build system to produce zip/tgz files with portable installation.

[Sébastien BLAISOT (sblaisot)]

Resources are automatically taken from a res subdirectory if it exists in the same location than the Mixxx executable.

For settings, it would be better to detect in some way (executable name ?) if we are running in portable mode and change default to a settings subdirectory in the same location than the mixxx executable. This way, we need no launcher and no option and have only one binary.

[Sébastien BLAISOT (sblaisot)]

https://github.com/mixxxdj/mixxx/pull/1780

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/8967