Fix compilation warnings

[cjfloss]

• use margin_start & margin_end instead of margin_left & margin_right
• Use elipsis instead of three dots
• fix elementary vala-lint warnings ('space-before-paren', 'use-of-tabs', 'trailing-whitespace')
• use set_accels_for_action instead of the deprecated add_accelerator
• use the correct verbatim string form (with three quotes)
• fix warning: use explicit .begin

fixes #22