Convert Components.ConversationActions to a widget that encapsulates
selectively visible conversations action groups. Use multiple instances
of that in preference to a single shared instance that gets re-parented
as the responsive UI adjusts.
Flatten the custom widget hierarchy a bit by merging the
`ConversationActionBar` and `ConversationHeaderbar` widgets into
`MainToolbar` itself.
HdyApplicationWindow moves the titlebar into the content area of the
window which allows us to split the current main-titlebar and move the
parts directly into the leaflets without the need of swipe groups nor
size groups.
This creates a new object that contain the 4 groups of actions that used
to be in the conversation-viewer headerbar.
This allows the widgets to be moved to differen locations, e.g. to an
action bar that will be added in a later commit.
Add Plugin.InfoBar to allow plugins to describe an info bar, add
methods to FolderContext to allow adding and removing them when
folders are displayed and implement them. Add a InfoBarStack to the
MainWindow for displaying folder info bars.
Add new `single-key-shortcuts` GSetting, Application.Configuration
property and add UI for it in the Preferences window. When enabled,
load in new `single-key-shortcuts.css` file that has the appropriate
bindings. Update bindings to match Google's current set, and update
shortcut help to match.
Move MainWindow out of the root namespace and add it to Application,
so it can privately share internal state with the Controller. Rename
the source and UI files to match.
