sidebar: switch subcategory header's DIY back button to be a ToolButton
requested to merge ngraham/systemsettings:switch-subcategory-header-diy-button-to-toolbutton into master
This is a more conventional UI element that fits just fine here. Due to a limitation of how QQC2.ToolButton aligns content when forced to be wide (see https://bugs.kde.org/show_bug.cgi?id=445033), we have to create our own custom content item for it. It looks fine with all tested color schemes as well as RTL mode.
cc @teams/vdg