Fix conflicts with switch to Hdy.HeaderBar

# Conflicts:
#   src/client/components/main-toolbar.vala

toolbar: Use HdyHeaderBar

Closes #1143

See merge request GNOME/geary!658

Fixes build warning introduced in commit c4fae242.

See GNOME/vala#1121

This was forgotten when porting to HdyWindow,
but is needed for making the window draggable.

main-window: replace GtkApplicationWindow with HdyApplicaitonWindow

See merge request GNOME/geary!656

Default attachment save file chooser to XDG_DOWNLOADS

See merge request GNOME/geary!654

Components.WebView: Fix printing to PDF with WebKitGTK sandbox enabled

See merge request GNOME/geary!655

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.

We know that location is writable under Flatpak at the moment, so
default to it.

We know that location is writable under Flatpak at the moment, so
default to it.

Add all of `~` as writable to the sandbox so it can actually write the
PDF.

Application.Controller: Fix directory used for locally pinned certs

See merge request GNOME/geary!652

Fix defect introduced in commit e8061379.

application-client: Check for null when getting os info

See merge request GNOME/geary!651

This caused a crash in the inspector system view when the os info wasn't
available.

Related to https://gitlab.gnome.org/GNOME/geary/-/issues/1123

Fix email being sent truncated after a line starting with a dot.

Closes #1126

See merge request GNOME/geary!647

Fixes #1126

Geary.Smtp.ClientService: Get default Message-Id domain from From

Closes #1079

See merge request GNOME/geary!644

Use From header as the default message id domain part, not Sender.

Fixes #1079

Composer keyboard fixes

Closes #1122 and #1059

See merge request GNOME/geary!643

Enable keyboard shortcuts shortcut in the composer, ensure the composer
section is visible when activated (and vice versa).

Fixes #1122

GTK widgets set the default for this as needed, and some incorrect
values for this have caused issues with keyboard focus.

Fixes #1059

Add private HeaderRow and EntryHeaderRow widgets for displaying editable
headers in the composer, convert to using that to wrap up common
properties and functionality for each of the header rows.

Conversation view: rounded containers

See merge request GNOME/geary!612

This also makes the expander row look like in HdyExpander row.

accounts-editor-add-pane: Autofill hostname

See merge request GNOME/geary!641

accounts-editor-add-pane: Fix bug on AddPaneRow constructor

See merge request GNOME/geary!642

AddPaneRow constructor created a new `Gtk.Entry` instead of using the
parameter provided.

Related to https://gitlab.gnome.org/GNOME/geary/-/issues/180