Add an option to set the custom color of text under the cursor

Hi,

This patch adds an ability to select the custom color of the character under the cursor (in addition to the custom color of the cursor itself). Please check the commit message for the details.

The picture below shows how custom cursor color looks in the current version of Konsole. Here I set custom cursor color to light gray. Observe that light green "k" character is hard to read under the light gray cursor:

With this patch one can set the custom character color to some contrast color, e.g. black. Looks much better:

Settings window before:

Settings window after: