GODT-1667: bridge-gui spawns bridge process. [skip-ci]

Other: renaming of bridge-gui.
WIP: locate bridge exe.
WIP: bridge process launch.
WIP: cleaner closure of bridge.
WIP: grpcClient connection retries.
WIP: clean exit when bridge process is killed.

Fixed issues from MR review. [skip-ci].

WIP: Fixed gRPC case in CMakelists.txt [skip-ci]

It caused issues on Debian.

WIP: update gRPC/protobuf and tweaked CMakeLists.txt. [skip-ci]

WIP: Fixed a bug where splash screen could not be dismissed. [skip-ci]

internal/frontend/bridge-gui/qml/Proton/qmldir

@@ -0,0 +1,37 @@
+# Copyright (c) 2021 Proton AG
+#
+# This file is part of Proton Mail Bridge.
+#
+# Proton Mail Bridge is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Proton Mail Bridge is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Proton Mail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+module QQtQuick.Controls.Proton
+depends QtQuick.Controls 2.12
+
+singleton ProtonStyle 4.0 Style.qml
+ColorScheme 4.0 ColorScheme.qml
+
+ApplicationWindow 4.0 ApplicationWindow.qml
+Button 4.0 Button.qml
+CheckBox 4.0 CheckBox.qml
+ComboBox 4.0 ComboBox.qml
+Dialog 4.0 Dialog.qml
+Label 4.0 Label.qml
+Menu 4.0 Menu.qml
+MenuItem 4.0 MenuItem.qml
+Popup 4.0 Popup.qml
+RadioButton 4.0 RadioButton.qml
+Switch 4.0 Switch.qml
+TextArea 4.0 TextArea.qml
+TextField 4.0 TextField.qml
+Toggle 4.0 Toggle.qml