Make HttpCredentialsGui::showDialog() modal

[TheOneRing]

The password dialog is non modal and thus can be "lost" beneath other windows.

See:
https://github.com/owncloud/client/blob/master/src/gui/creds/httpcredentialsgui.cpp#L115

My suggestion would be to add a singleton to access the SettingSialog from everywhere and thus provide a parent for that dialog.

[TheOneRing]

@ogoffart what do you think about that singleton?

[ogoffart]

Yes, a singleton makes sense for this.
I believe there is already a ownCloudGui singleton that could be used to access the settings.
Note that the settings is not always visible.