Add helper function variant_string

It is used to extract string from variant with unboxing and data type
checking.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Use variant_string in variant_to_strv

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add size and type checking to Form specs

It now throws FormError on invalid form spec. Example form updated.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Fix label of bool form field

Id field was used for a label instead from label field.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>