Create application icon

[jaragunde]

It would be great to create an application icon, something that we could show in the header and use as favicon or home page icon.

I've found this service that would help with the multi-browser setup of favicons, ideally we'd like that to work in as many browsers as possible: https://realfavicongenerator.net/ (found at stackoverflow).

[jaragunde]

Reviewed in #386.

commit fd3ed9418d911d4894b3e211c05db89cbcd9c9cb
Merge: 93951dce 3f524671
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Fri Mar 2 11:40:56 2018 +0100

    [#381] Add application icon
    
    Create application icon and use it in the header bar, as a favicon,
    tile icon, shortcut icon...
    
    (Merge pull request #386 from Igalia/i381-application-icon)

commit 3f5246712e93b085c657120d7bb0797af649ed76
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Tue Feb 20 20:00:59 2018 +0100

    [#381] Add HiDPI version of header icon.

commit d30c0f18bd2d42f7cbfbd47dfc3d3710b51e2afe
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Fri Feb 9 14:30:15 2018 +0100

    [#381] Configure favicon and shortcut icons for diverse platforms.
    
    Configuration and icons generated automatically with:
    https://realfavicongenerator.net/

commit b5c3c1d9448b96d7af8b8be2b59afe9a1c3896be
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Fri Jan 19 20:10:05 2018 +0100

    [#381] Add header icon to the menu bar.
    
    Icon generated from application-icon.svg using a light grey background
    instead of while, it seems to look better with the page background
    gradient.

commit 5bbe2692f3dd42cc1817c4b196da2016dddae08d
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Fri Feb 9 13:59:00 2018 +0100

    [#381] Add application icon in .svg format.