Form: set spacing on boxes, use header label features

[danirabbit]

• Set spacing on boxes instead of individual widgets
• Set mnemonic widgets for entries so that are labeled for the screen reader
• Use secondary_text feature of HeaderLabel for additional description text. This makes sure it's read by the screen reader and that we aren't hiding helpful info
• Use placeholder feature of entry instead of "such as"

[alainm23]

It looks cleaner, thanks.