SelectableLabel: Prevent focus on press as well
Focus on Tab proved to be useless, so it was disabled. There isn't much reasons to accept focus on mouse/touch press either. Especially when used inside popup dialogs, it's better to keep focus on dialog buttons instead.
Similar to !1063 (merged), but this time it's controversial: usually if you select text by mouse you'd expect to be able to copy it. But since this commit disables focus on press, keyboard shortcut Ctrl+C won't reach our text area, and as we still lack a context menu in qqc2-desktop-style, you can't even right click and select "Copy" menu item — it's only implemented for TextField, not for TextArea.