GODT-1554 / 1555: Implement gRPC go service and Qt 5 frontend C++ app.

WIP: updates

WIP: cache on disk and autostart.

WIP: mail, keychain and more.

WIP: updated grpc version in go mod file.

WIP: user list.

WIP: RPC service placeholder

WIP: test C++ RPC client skeleton.

Other: missing license script update.

WIP: use Qt test framework.

WIP: test for app and login calls.

WIP: test for update & cache on disk calls.

WIP: tests for mail settings calls.

WIP: all client tests.

WIP: linter fixes.

WIP: fix missing license link.

WIP: update dependency_license script for gRPC and protobuf.

WIP: removed unused file.

WIP: app & login event streaming tests.

WIP: update event stream tests.

WIP: completed event streaming tests.

GODT-1554: qt C++ frontend skeleton.

WIP: C++ backend declaration.

wip: started drafting user model.

WIP: users. not functional.

WIP: invokable methods

WIP: Exception class + backend 'injection' into QML.

WIP: switch to VCPKG to ease multi-arch compilation,  C++ RPC client skeleton.

WIP: Renaming and reorganisation

WIP:introduced new 'grpc' go frontend.

WIP: Worker & Oveerseer for thread management.

WIP: added log to C++ app.

WIP: event stream architecture on Go side.

WIP: event parsing and streamer stopping.

WIP: Moved grpc to frontend subfolder + use vcpkg for gRPC and protobuf.

WIP: windows building ok

WIP: wired a few messages

WIP: more wiring.

WIP: Fixed imports after rebase on top of devel.

WIP: wired some bool and string properties.

WIP: more properties.

WIP: wired cache on disk stuff

WIP: connect event watcher.

WIP: login

WIP: fix showSplashScreen

WIP: Wired login calls.

WIP: user list.

WIP: Refactored main().

WIP: User retrieval .

WIP: no shared pointer in user model.

WIP: fixed user count.

WIP: cached goos.

WIP: Wired autostart

WIP: beta channel toggle wired.

WIP: User removal

WIP: wired theme

WIP: implemented configure apple mail.

WIP: split mode.

WIP: fixed user updates.

WIP: fixed Quit from tray icon

WIP: wired CurrentEmailClient

WIP: wired UseSSLForSMTP

WIP: wired change ports .

WIP: wired DoH. .

WIP: wired keychain calls.

WIP: wired autoupdate option.

WIP: QML Backend clean-up.

WIP: cleanup.

WIP: moved user related files in subfolder. .

WIP: User are managed using smart pointers.

WIP: cleanup.

WIP: more cleanup.

WIP: mail events forwarding

WIP: code inspection tweaks from CLion.

WIP: moved QML, cleanup, and missing copyright notices.

WIP: Backend is not QMLBackend.

Other: fixed issues reported by Leander. [skip ci]

internal/frontend/qt6/qml/icons/Loader_48.svg

@@ -0,0 +1,4 @@
+<svg width="48" height="48" viewBox="0 0 48 48" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path opacity="0.2" d="M24 1C27.0204 1 30.0112 1.59491 32.8017 2.75077C35.5922 3.90663 38.1277 5.6008 40.2635 7.73655C42.3992 9.87229 44.0934 12.4078 45.2492 15.1983C46.4051 17.9888 47 20.9796 47 24C47 27.0204 46.4051 30.0112 45.2492 32.8017C44.0934 35.5922 42.3992 38.1277 40.2635 40.2635C38.1277 42.3992 35.5922 44.0934 32.8017 45.2492C30.0112 46.4051 27.0204 47 24 47C20.9796 47 17.9888 46.4051 15.1983 45.2492C12.4078 44.0934 9.87228 42.3992 7.73654 40.2634C5.60079 38.1277 3.90662 35.5922 2.75077 32.8017C1.59491 30.0112 0.999998 27.0204 1 24C1 20.9796 1.59492 17.9888 2.75078 15.1983C3.90664 12.4078 5.6008 9.87228 7.73655 7.73653C9.8723 5.60079 12.4078 3.90662 15.1983 2.75077C17.9888 1.59491 20.9796 0.999998 24 1L24 1Z" stroke="#5064B6" stroke-width="2"/>
+<path d="M40.2635 40.2635C37.5889 42.938 34.2961 44.9117 30.6765 46.0096C27.057 47.1076 23.2226 47.296 19.5129 46.5581C15.8032 45.8202 12.3328 44.1788 9.40895 41.7792C6.48514 39.3797 4.19822 36.2962 2.75077 32.8017C1.30332 29.3073 0.740014 25.5098 1.11075 21.7456C1.48149 17.9814 2.77483 14.3668 4.8762 11.2219C6.97757 8.07696 9.82212 5.49881 13.1579 3.71581C16.4936 1.93281 20.2176 1 24 1" stroke="#5064B6" stroke-width="2"/>
+</svg>