Use icons-only "Show Sidebar" Button with new icon, if available

This commit makes the "Show Sidebar" button that's on the toolbar by
default use the new `sidebar-expand` icon, if present. If not, it falls
back to the old `view-sidetree` icon for compatibility's sake.

We also make thie button icons-only by default, as the new icon is
clearer than the old one. This saves some horizontal space in the
defualt toolbar, which is now quite wide.

part/part.cpp

@@ -861,7 +861,8 @@ void Part::setupActions()
 
     m_showLeftPanel = ac->add<KToggleAction>(QStringLiteral("show_leftpanel"));
     m_showLeftPanel->setText(i18n("Show S&idebar"));
-    m_showLeftPanel->setIcon(QIcon::fromTheme(QStringLiteral("view-sidetree")));
+    const QString preferredSidebarIcon = m_sidebar->layoutDirection() == Qt::LeftToRight ? QStringLiteral("sidebar-expand-left") : QStringLiteral("sidebar-expand-right");
+    m_showLeftPanel->setIcon(QIcon::fromTheme(preferredSidebarIcon, QIcon::fromTheme(QStringLiteral("view-sidetree"))));
     connect(m_showLeftPanel, &QAction::toggled, this, &Part::slotShowLeftPanel);
     ac->setDefaultShortcut(m_showLeftPanel, QKeySequence(Qt::Key_F7));
     m_showLeftPanel->setChecked(Okular::Settings::showLeftPanel());

part/part.rc

@@ -1,6 +1,6 @@
 <?xml version="1.0"?>
 <!DOCTYPE gui SYSTEM "kpartgui.dtd">
-<gui name="okular_part" version="48">
+<gui name="okular_part" version="49">
 <MenuBar>
   <Menu name="file"><text>&amp;File</text>
     <Action name="get_new_stuff" group="file_open"/>
@@ -110,6 +110,9 @@
   <Action name="zoom_to" />
   <Action name="view_zoom_in"/>
 </ToolBar>
+<ActionProperties scheme="Default">
+  <Action priority="0" name="show_leftpanel"/>
+</ActionProperties>
 <ToolBar name="annotationToolBar" iconText="icononly" newline="true" hidden="true"><text>Annotation Toolbar</text>
   <Action name="annotation_highlighter"/>
   <Action name="annotation_underline"/>