Fix for U4-3953 (modified)

I overlooked the installing message, so here the message is still
visible below the progressbar. Furthermore the same is fixed for the
uninstalling progressbar, which I didn't noticed previously.
Merge branch '7.2.0' of https://github.com/bjarnef/Umbraco-CMS into bjarnef-7.2.0

Conflicts:
	src/Umbraco.Web.UI/umbraco/developer/Packages/installer.aspx
Merge branch 'bjarnef-7.2.0' into 7.2.0
typo