Modernize gallery example
Modernize gallery example

  - Destructure tabris import to make the code more compact and to avoid
    using the global tabris object.
  - Use let instead of const where variables are not "typical" constants
    (e.g. storing primitives). It may be a good practice to use const
    over let when possible, but it is also 2 characters longer and this
    practice may be an overmeasure in the scope of Tabris.js examples. Also
    be consistent with other examples.
  - Use image shorthand when possible. When only source is given
    (without other image properties), it can be given directly as a
    string.

Change-Id: I3c42a9f0d260b723485e1ee43f2a7b8c491cb2e3

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js