Fix the preference panes each using their own layout and sizes - #673

The panes drifted apart over time: Integration and Updates sat on
different left columns from General, several controls used the small
system font while their neighbours used the regular one, and the
margins were tight enough that every pane looked crowded.

• Put all three panes on the same columns, with sub-options indented
  one step under the option they belong to
• Move the seven controls that still used the small system font onto
  the regular one, at the regular control size
• Separate the terminal setting, the Gist options and Enable Gravatar
  from each other with vertical space, and widen the top and bottom
  margins of every pane by the height of a line of text
• Center the Check Now button, which acts for the whole Updates pane

The explanatory text under Git Executable keeps the small size, which
is what that size is for.

The last-update date is the one change here that is not layout: at the
regular font size the full style measures 296pt in en_US against the
265pt its row has, so it would clip. The long style measures 219pt
and fits, at the cost of the weekday.

Screenshots

'General' pane

Before:

After:

'Integration' pane

Before:

After:

'Updates' pane

Before:

After:

Test plan

• ibtool --errors --warnings --notices reports no errors and no new
  clipping, only the same intrinsic-height notice that every other
  label in this xib already carries
• Built and run: the accessibility tree reports the shared columns on
  all three panes (options at 104, sub-options at 125, controls at
  219), regular-size controls, and Check Now centered at 249.5 in a
  500pt pane
• Open Preferences and switch between the tabs to check that labels
  and controls do not shift sideways