Cleanup unused .h
Cleanup default widgets
Use a smaller size of toolbuttons
Fine-tune default widgets' layout of toolbox
Remove unused code
Reorder redo and undo buttons
Format code
Enable flexible UI layout (horizontal)
Use small icon size for dock panel title
Cleanup size related code and add some inline notes.
Place gradient widget on top of interpolation widget
Remove unneeded code
Enable flexible row spacing for toolbutton in toolbox.
Make defaults widgets UI layout more elegant

Make fill and outline color widgets have a fixed size,
Make brush preview widget has a fixe size,
Make the other defaults widgets have flexible width,
Make defaults widgets have flexible row spacings.

Swap and Reset buttons should be fine-tune as well, temporarily commented out.
Use standard size of toolbar of gtk+ for global toolbar
Change default toolbutton size and disable vertical auto spacing of toolbar.

Use standard small toolbar size of gtk+
Disable row auto spacing of toolbutton
Fine-tune row spacing of toolbox elements

Use standard small size of toolbar of gtk+,
Use same codding type
Use tiny size of icon for swap and reset buttons.
Merge branch 'jcome/toolbox-toolbar'

* tmp:
  Use tiny size of icon for swap and reset buttons.
  Use same codding type
  Change default toolbutton size and disable vertical auto spacing of toolbar.
  Use standard size of toolbar of gtk+ for global toolbar
  Make defaults widgets UI layout more elegant
  Enable flexible row spacing for toolbutton in toolbox.
  Remove unneeded code
  Place gradient widget on top of interpolation widget
  Cleanup size related code and add some inline notes.
  Use small icon size for dock panel title
  Enable flexible UI layout (horizontal)
  Format code
  Reorder redo and undo buttons
  Remove unused code
  Fine-tune default widgets' layout of toolbox
  Use a smaller size of toolbuttons
  Cleanup default widgets
  Cleanup unused .h
Revert "Use standard size of toolbar of gtk+ for global toolbar"

This reverts commit 7d98d0b5f64fb1c145127b34bb07cc2e5b359c86.