Multiple UI tweaks.

New features/tweaks:
* Directory with docsets is followed recursively (including symlinks). Allows to have docsets split across many folders/devices.
* Adds UI layout changes. Adds homepage URL. Allows to open external links in a default browser.
* Adds prefix autocompletions: For example after installing nginx typing "ng[tab]" would autocomplete to "nginx:"
* Adds history menu: The history is now easy to navigate through.

Issues:
* #62: Adds '/' and '?' keyboard shortcuts.
* #129: When the search box in the browser window is focused ESC will close the search box and not clear the main search.
* #130: Adds an outline pane. Does not work if the links are embedded in HTML (C, C++)
* #140: Fixes the displayed name of methods which contain ellipses in their arguments.
* #156: The search box should correctly get focused after a single click.
* #159: QT >= 5.2.0 will use a command line parser. This includes the --help flag. Also adds a --force flag which allows to open multiple instances of zeal at once.
* #166: If CFBundleName is present it will be used as a display name.
* #178: Allows to create, delete, select multiple tabs. Adds appropriate keyboard shortcuts to simplify navigation.
* #186: RFC docset will be displayed with a correct name (related to bug #166). Docsets downloaded from an external URL do not have icons present thus the icon will remain empty.