Support Linux platform

[rclement]

Meeblip Controller does not support the Linux platform yet. As for MacOS and Windows, the Linux version should be available as a plug-in (VST2, VST3) and a standalone application.

Main changes should concern the following:

• CMake configuration
• Installer definitions
• CI scripts
• Travis-CI build and deploy
• AppVeyor build and deploy

About the installer, distribution for the various Linux distributions is still an open question. AppImage and Snap seem to be getting popular, user-friendly and available for all Linux distributions. Suggestions are welcome!

@johannesgunnar: feel free to jump in!

[rclement]

Work in progress in the feature branch linux-support.

[rclement]

Quick heads-up on the linux-support branch:

• I managed to fix the Linux build both on my end and Travis-CI server
• Building on Linux requires the use of Clang instead of GCC
• The standalone app version seems to be fine
• I had to disable the use of ALSA (for external MIDI device support) for the VST version not to crash at startup. Unfortunately, disabling ALSA is all or nothing, so this makes the standalone app un-usable...

What needs to be done:

• Try to fix the ALSA issue (either disable it only for VST or find another solution to avoid the segfault)
• Package the standalone application as an AppImage
• Bundle everything (standalone app, VST plugin, factory presets, etc.) with a simple installer

[rclement]

Quick note on the ALSA issue (for further reference):

• Related issue with Dexed plugin
• Cherry-picking this commit does not fix the problem, the code is still crashing when calling snd_seq_set_client_name

[rclement]

Linux support has been merged into develop branch: fa57dfb

Features:

• Standalone application as an AppImage
• VST plugin (tested with REAPER)
• Optional minimal installer/uninstaller putting everything into place (AppImage, VST plugin, factory presets, licenses)

Known limitations:

• The Standalone app is currently "unusable" because of the ALSA issue
• The VST plugin is usable but only in host mode (using the external MIDI device does not work because of the ALSA issue)

@johannesgunnar: you can give it a try if you want to! For now you have to build from the develop branch but the Linux installer will be embedded with the next release!

[probonopd]

Please provide the AppImage outside of a zip, thank you. More information: https://github.com/AppImage/AppImageKit/wiki/Creating-AppImages#common-mistake