Change the way saving and loading profile works (drop lastly used in favor of explicit default)

Closes: #90 (closed) and #91 (closed)

Previously we had a feature of always loading lastly used profile if no explicit profile name was given.

This had some side effets because e.g when creating new profile - it was already set as lastly used.

It was really noticeable in the CLI and might be annoying that operating on another profile always changed the default one (lastly used) like:

<profile is alice>
clive show profile --profile-name other_profile
<profile is other_profile>

We dropped that behavior in favor of explicitly setting a default one (first profile is set as default also, but saving other profiles won't modify the default to load, until ProfileData.set_default_profile is called explicitly) and now it works like:

<profile is alice>
clive show profile --profile-name other_profile
<profile is alice>
clive configure profile set-default --profile-name other_profile
<profile is other_profile>