The {group,entry}CloseButton QToolButton, had the "checkable" property set.
This caused it to act like a toggle flip flop instead of a momentary push
button. After removing that property, the signal it was changed to use
was clicked() instead of toggled(bool). Trigger upon click is
consistent with the rest of the UI's momentary buttons.
Rename UI elements involved with the bottom preview panel to use the
terminology "entry preview panel" instead of "details view" or "preview
panel". Change all associated variables, widgets, and classes to
reference EntryPreviewPanel. Create Config::upgrade() function and
deprecation mapping to help rename/remove previous config settings in a
clean manner.
Fixes#2327