Fix Qt4 figure editor color setting and getting

This fixes #2274. It also permits only MPL colors to be entered in the color lineedit boxes. Previously, it seems Qt4 was dealing with any named colours entered into the lineedits, instead of MPL. This probably came out the same most of the time, but was less than ideal. Now, all colors are passed through MPL, which returns an unambiguous hex string for Qt to use. Also, there was some code duplication in terms of checking for valid colors. Aslo, when editing the color lineedits, the corresponding color buttons now only update when the lineedit loses focus, not on every keypress. Finally, the mass of Qt objects entered directly into the namespace have been cleaned up.