Make FoldersTabsSettingsPage fully usable for blind users
This commit fixes keyboard navigation for the "Folders & Tabs" page in Dolphin's settings dialog. It also makes sure that the Orca screen reader reads out sufficient information for every widget that can get keyboard focus. These two groups of fixes together should allow basic usage of this settings page for users relying on keyboard controls and screen readers.
One weirdness prior to this commit was that for checkboxes with a short group label in front and the full description behind screen readers would only read out the label and skip the explanation what the checkbox actually does. This commit fixes this by explicitly setting the accessible text for the label. In practice only reading out the explanation and skipping the label seemed sufficient and is implemented here, but more specific text could be provided using this mechanism if needed.