Update styles previews in Idle

Update styles previews in Idle

This helps with performance when we do series of actions
updating styles. In that case we will only request update.
When we will reach idle state we will do it only one time.
What removes unnecessary updates in the meantime.

Change-Id: I9fc59992833a6cf98c42571c1189b3c5d49ba5a0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/126840
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130232
Tested-by: Jenkins
Reviewed-by: Szymon Kłos <szymon.klos@collabora.com>