Add blend method widget into gradient tool options panel.
Improve UI layout of Options.

1. Options Panel border width set to 3px
2. Row spacing is 5px
3. Widgets spacing in the same row is 3px
4. Spacing of sub-option widget is 8px
Add(uncomment) blend method wiget for Circle Tool Options Panel
Add opacity widget to circle tool options panel.

It is still a global wide widget, should be changed to a local option which
affects circle tool only.
Add brush size widget to circle tool options panel.

It is still a global wide setting, should be changed to affect
circle tool only.
Remove comment
Replace checkbutton with togglebutton for layer creation
Improve labels and add some missed labels
Pack all layer type in a hbox

- Pack all layer creation buttons in a hbox,
- Pack  Name and its text field in an other hbox,
- Align Title label to left.
Align Checkbutton items

Checkbox at the right and its label at the left
Don't expand layer creation buttons.
Order options
Set border width of circle tool options panel, 3px.

Set 10px spacing to layer creation buttons row, let each row has same hight.
Add a space to ID(Name) Label between Id Entry
Fine-tune options layout
Use standard icon size

Use Gtk::ICON_SIZE_SMALL_TOOLBAR for toolbox, global toolbar and dock panel title icon.
We should consider define a guideline for icon sizes using in Studio UI.
Add layer type (4 Gradient layer types) creation buttons.

Their funcition is not implemented yet.
Typo: lineal -> linear
Don't share blend method option of gradient tool with whole Synfig Studio.
Use a more compact UI layout.