Use ESCET logo in relevant places

We're close to having a logo in #777 (closed). Once we have it, we should use it in the appropriate places. For now, I foresee the following places:

• ESCET website (!838 (merged))
  • ESCET logo on ESCET home page.
  • ESCET favicon for the ESCET static website pages (the yellow pages), like already there for CIF, etc.
  • ESCET favicon for the ESCET general toolkit documentation and the development documentation, like already there for other docsets.
  • Add icons in the site selector dropdown (the one from the top bar, on the very left enty of that bar, to select different websites ESCET/CIF/etc).
• ESCET IDE (!840 (merged))
  • Change the splash screen image.
  • Change the about screen image.
  • Change the welcome screen image.
  • Change the icon of the ESCET perspective.
  • Change the 'feature image'.
  • Change the 'window images'.
  • Add an executable icon (for Windows). We currently don't have one yet, and thus get the generic Eclipse icon.
• Project website / PMI
  • Add the logo to https://projects.eclipse.org/projects/technology.escet. See also https://www.eclipse.org/projects/handbook/#promoting-project-logo.