The TagCombo is not loaded, when requesting category color

Summary:
TagCombo sometimes returns as selectedIndex -1, when running
KOPrefsDialogColorsAndFonts::usrReadConfig(). In this case we wait for
currentIndexChanged signal of the CategoryCombo.

Reviewers: mollekopf

Maniphest Tasks: T1274

Differential Revision: https://git.kolab.org/D176