mainToolbar: Do not use a label for back button

Back button just shows an icon. See 706908
searchBar: use GTK_STYLE_CLASS_PRIMARY_TOOLBAR

Do not directly use the string "primary-toolbar"
icons: do not install the source svg

Install all public hicolor apps icon but this one. Put this to no inst.
See 706910
mainToolbar: remove New button padding

But stick to size request. See 706911
mainToolbar: Show selection mode label as primary label

Do not use subtitle for the selected items label
See 706912