Update QgsColorButton so it shows a color chooser on click
Update core components to use the new QgsColorButton implementation
[FEATURE] Update QgsColorButton to offer live color chooser updates and show transparent checkerboard background

- Add option to Options for live color chooser support (QgsColorDialog)
- Add ability to define whether QgsColorButton accepts live updates (default: true)
- Move QgsColorButton to single subclass of QPushButton
- Show different button types relative to whether button has text
- Add transparent checkerboard background for chosen colors with alpha < 255
- Fix triple-modal window issue for Mac (with regards to using native color dialog)

- Composer item frame now supports transparency
- Composer item background transparency support moved to color dialog
- Composer composition grid now supports transparency
Fix missing conversion from QgsColorButtonV2 to QgsColorButton
Fix for #7398, allow alpha value change in line pattern fill color picker

- Add alpha picker and render support for: line pattern fill, line decoration symbol, font marker symbol
Merge pull request #475 from nyalldawson/blend_modes

Add support for blending (composition) modes, eg Overlay, Mutiply, etc (Close #5248)
Fix live color updating slot parameter by using SLOT macro
Fix parsing bug in wkb -> geos export
Merge pull request #474 from dakcarto/color-button_mk_10

[FEATURE] QgsColorButton shows a color chooser dialog on click, checkboard color when translucent, and live updates during color choosing
Move blend mode combo box to styles tab
Add support for copy/pasting blend modes for layers