ThickClient: adapt classes to have static SchemaEntries (GSettings friendly)

PersistentWindowController class had schema entries created at runtime
which meant that the classes dependent on this GUI logic could not be
inspected by GSettingsSchemaExtractor (static analysis) to find their
configuration properly.

With this change, we abstract this set of SchemaEntry instances related to
window configuration (posizion, size, maximized|minimized) into a new class
called WindowConfiguration, which has also some static helpers to avoid
repeating the summary, description and default values of all the schema
entries that have the same info.