AliasesDialog: screen reader fixes, design tweaks

[danirabbit]

Alias:

• Use a vertical form layout with header labels to match the design of other forms
• Make sure the screen reader knows how to describe entries by setting mnemonic_widget
• Placeholder text in forms should contain example content, not explanation text or directions
• Image buttons should have tooltip text (screenreader reads this too)

AliasDialog:

• Subclass Granite.Dialog since we're not doing anything that requires a Hdy.Window
• Dialogs should have a transient parent so they are centered on the parent etc
• Make sure screen reader can read the add button
• Use built-in method for dialog buttons instead of adding window controls manually