Accessible from main menu Help > About Meshroom, or by pressing "F1". Provides info about Meshroom version + useful links and related Open Source Licenses.
* load COPYING.md from Meshroom/AliceVision files (either provided with the standalone package or online) + translate them to HTML for display (added `markdown` package in requirements)
* request.js: centralize function to open file urls
* provide system info from MeshroomApp (exposed as contextProperty)
