Use built-in styles, improve screenreader

[danirabbit]

Some proposed changes to the popover

• Use MENU style. Saves us a bunch of manually setting of margins, padding etc
• Remove the icon from the header and use Granite.HeaderLabel. More clearly visually separates the header from the list of apps stylistically
• Make the secondary text of the header a little more concise
• Set the header as mnemonic widget for the list so the screenreader associates it and reads it correctly
• Make secondary text for app rows small text to give more visual hierarchy and make more room for long labels
• Change tooltip for button to "Quit" to be more concise
• Make buttons destructive styled and slightly smaller

AFTER

BEFORE

[leolost2605]

Much better, thank!