add logic to disk view partition for button display
add logic to disk view partition for button display

Depending on if partitions already exist or not will determine
if the add partition label shows the first gpt partition or just
a plain "add partition" label. For create swap it is disabled
and the color dimmed if partitions already exist.

Signed-off-by: Adam Stokes <0a364f4bf549cc82d725fa7fd7ed34404be64079@ubuntu.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity