Added an ability to pick colors from the Color To Alpha filter

This patch makes several quite significant changes:
1) KisToolPickColor now saves (!) its configuration to the disk! So you'll
have your options loaded on the next run of Krita
2) KisToolPickColor now has two sets of settings: for temporary activation
(the one used in filters) and for usual activation. They both are
load/save'able.
3) The KoToolManager's methods for temporary switching of the tool are now
made public. I guess, they were made private initially, because there was
no usecase for it those days.

So the interaction with the color2alpha filter is the following:

1) You open a filter
2) The filter activates the picker tool
3) You can pick colors and change options of the tool: these changes will
not affect your normal settings
4) When you press Ok/Cancel the original tool is reactivated

Known bugs:
Color Picking doesn't work with Adj. Layers and Filter Masks :(