[FEATURE] New custom QGIS color picker dialog. Basically

it's a Frankenstein's monster of the best bits of lots of other color
picker implementations. Features include:
- Sliders for hue, saturation, value, red, green, blue color components
- Alpha slider
- Option to copy/paste html color codes (in a variety of formats)
- Before/after color preview, with alpha preview
- Interactive 2D color ramp widget
- Hue wheel/triangle widget
- Editable color swatch lists
- Live color sampler with option to sample over a set radius
- Supports interaction with other apps via drag and drop of colors (OS support
dependant)
- Option to disable the dialog and use the native picker dialog instead (TODO:
make the dialog so awesome that even @dakcarto switches across...)

Credit for icons goes to 3nids!

images/images.qrc

@@ -294,6 +294,10 @@
         <file>themes/default/mIconClear.svg</file>
         <file>themes/default/mIconClose.png</file>
         <file>themes/default/mIconCollapse.png</file>
+        <file>themes/default/mIconColorBox.svg</file>
+        <file>themes/default/mIconColorPicker.svg</file>
+        <file>themes/default/mIconColorSwatches.svg</file>
+        <file>themes/default/mIconColorWheel.svg</file>
         <file>themes/default/mIconConnect.png</file>
         <file>themes/default/mIconCritical.png</file>
         <file>themes/default/mIconDataDefine.svg</file>