qt branch merge

This is a umbrella issue to triage and track the blockers to merge the qt branch. We should link individual issue numbers from here.

Blockers

• #299 (closed) fix event handling segfault
• #243 (closed) migrate i18n to qt

Essential

• implement dialogs !100 (merged)
• open links with qt (donate, about, ...) !100 (merged)
• error dialogs
• #279 (closed) evaluate qt installer for win/osx
• #297 (closed) modify snap to use qt
• #296 (closed) integrate build.sh into makefile
• osx: ensure window is not visible by default
• #328 (closed) produce qt installer for osx
• #327 (closed) produce qt installer for windows

Can be deferred, but related to this change

• properly handle failed state (firewall up) in the systray
• #289 (closed) re-implement cli flags
• docker-based cross-compiling for snap
• #298 cross-compiling for win: mxe
• windows build: mxe vagrantfile/dockerfile
• use qtlite to minimize bundle size
• gnome workaround: detect systray missing
  • #322 use a visible window as main control
• #314 (closed) automate transifex pushes
• cross-compiling for osx: osxcross
• #326 (closed) refactor branding mechanism