Use full pathname for Icon in .desktop for snap (#25)
This change adds a `DESKTOP_ICON` variable in CMake to customize the value of `Icon` in the `.desktop` file, which in the case of the snap should be an absolute filename.
The **unholy trinity** of errors concerns plugins, redirects and downloads. Most typically it surfaces like so:
- The address typed in the urlbar leads to a downloaded file, and the tab shows an error.
- A loading web page is cancelled via Esc or the toolbar button.
Implement speed dial based on recent history (#15)
![screenshot from 2018-08-18 20-45-09](https://user-images.githubusercontent.com/1204189/44302400-03a58a80-a328-11e8-8c4f-88be485e7920.png)
The speed dial is implemented as an "internal" scheme handler, which won't require special-casing on the UX side except using "internal:speed-dial" as a default URI in Midori.Tab and recognizing it as "empty" in Midori.Urlbar.
All items are populated automatically from the history and use favicons.