MainView: justify layout, space between sections

[danirabbit]

Try to make this layout a bit less wacky and easier to maintain:

• Put sections in their own grids and double space between sections
• Justify layout of widgets and labels
• Set mnemonic widget where possible (Some comboboxes will need this fixed when ported to dropdown)
• Put infobar by the lid settings

[jeremypw]

@danirabbit Could you resolve the conflict please?

[danirabbit]

@jeremypw done!

[jeremypw]

Code and appearance look good, but I am getting a system hang (unresponsive to keyboard and mouse) when the authentication dialog appears after selecting a lid close option. Not sure whether due to this PR though.

Update: It does not occur in main.

[danirabbit]

@jeremypw this branch only touches layout code. There are no functional changes in this branch whatsoever

[jeremypw]

Approving on the basis that it only affects layout, not function.