[Color Picker] Try to parse color from text drop

Summary: When dropping a color code from an text editor, try to figure out the color and add it to the history.

Test Plan: Dragged a couple of color strings from Qt's documentation and it worked. Dropping colors from Inkscape and Kolourpaint still works.

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5246