Extended FAB: Enable padding customizations

[kfranqueiro]

After discussing with designers, the baseline extended FAB will consist of the following:

• 12px icon-side padding
• 24px icon
• 12px padding between icon and label
• 20px label-side padding

The baseline for extended FAB with no label will have 20px padding on each side.

We need to be able to customize the following:

• The basis for the icon-side padding and padding between icon and label (both of these should equal each other)
• The label-side padding

Additional thoughts:

• It might be nice if we could bundle icon size and padding adjustment to compute the padding automatically depending on the size, but this might warrant prototyping to see whether "smarter" is actually easier to use
• We'd still need to keep in mind that we don't need to adjust padding for the default collapsed FAB (we simply center the icon, no matter the size)
• We also need to properly style icon-less extended FAB to have equal padding on both sides (using the label-side padding setting)