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