feat(ui): give Sync the refresh icon

The Sync button used mail-receive, a mailbox glyph, which reads as "mail"
rather than "fetch again". The toolbar follows the desktop's tool button
style, so on an icon-only desktop the icon is the whole control and has to
carry the meaning by itself.

view-refresh is the standard freedesktop name for the action. The existing
noTwoActionsShareAnIcon test covers the collision risk that the 0.12.0
Archive/Mark-all-read defect came from, and passes.

Also records the backlog reconciliation this came from: items 64 and 65,
appended from the user's notes with their causes verified in code. 65 is
"full code review and optimization", which names no symptom or measurement
and is filed unspecified rather than given a design.

Backlog item 64.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
index bc5fa92..d9eb989 100644
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -997,7 +997,7 @@ void MainWindow::buildMenus()
     // rather than taken from the spec on faith. A name the running theme lacks
     // still degrades to text through the null check below.
     const QHash<QString, QString> themeIcons = {
-        { QStringLiteral("sync"),    QStringLiteral("mail-receive") },
+        { QStringLiteral("sync"),    QStringLiteral("view-refresh") },
         // NOT mail-mark-read, which mark_all_read below uses. The two shared it
         // in 0.12.0, and with the toolbar icon-only the icon is the whole
         // control: two buttons with different consequences looked identical.