Make FoldersTabsSettingsPage fully usable for blind users

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.