Fix multiple issues in colors KCM

- do not offer "Current" (or "Default") name for saving
- update preview icon when a colors gets changed
- write "Current" as scheme name, when the temporary theme is used
- upon loading the "Current" scheme, mark it as "needs saving"
- update preview icon for "Current" scheme when it is applied

See review description for details.

REVIEW: 106066
BUG: 305350
FIXED-IN: 4.9.1