EMemoShellSidebar: Use EClientCache to determine refresh support.

In memo_shell_sidebar_check_state(), determine whether the selected
source supports refresh by way of e_client_cache_ref_cached_client()
and e_client_check_refresh_supported().
ETaskShellSidebar: Use EClientCache to determine refresh support.

In task_shell_sidebar_check_state(), determine whether the selected
source supports refresh by way of e_client_cache_ref_cached_client()
and e_client_check_refresh_supported().
ECalShellSidebar: Remove internal client table.

No longer needed, replaced by EClientCache.
EMemoShellSidebar: Remove internal client table.

No longer needed, replaced by EClientCache.
ETaskShellSidebar: Remove internal client table.

No longer needed, replaced by EClientCache.
Updated Uyghur translation

Signed-off-by: Gheyret Kenji <gheyret@gmail.com>
Updated Galician translations
Updated Polish translation
EventPage: Use EClientCache to obtain an EClient.
MemoPage: Use EClientCache to obtain an EClient.
TaskPage: Use EClientCache to obtain an EClient.
EBookShellBackend: Use EClientCache to obtain an EClient.
EBookShellView: Use EClientCache to obtain an EClient.
ECalShellBackend: Use EClientCache to obtain an EClient.
EMemoShellBackend: Use EClientCache to obtain an EClient.
ETaskShellBackend: Use EClientCache to obtain an EClient.
EClientCache: Keep a strong reference on the ESourceRegistry.

Changed my mind.  There's some scenarios where a function depends on an
ESourceRegistry but could also utilize an EClientCache, and it would be
nice to just pass the EClientCache and extract the ESourceRegistry from
it, so we need to make sure the ESourceRegistry will be there when it's
needed and not quietly disappear.
NEWS update for 3.7.90 release.
Post-release version bump.
Updated Serbian translation