Make SDL window resizable and optionally fullscreen

[jankammerath]

F key should make it go fullscreen and in window mode the window should be resizable.

[jankammerath]

I implemented fullscreen toggle. In the SDL documentation it describes how to define a window as resizable: https://www.libsdl.org/release/SDL-1.2.15/docs/html/sdlsetvideomode.html

The problem however with the SDL_RESIZABLE flag is that it signals the resize event which then requires all elements (LibVLC, WebKit2Gtk and SDL) to resize theirselves. Also patched issues where the dimensions defined in config were not considered by WebKit.

However all of this work does not reflect the initial ticket purpose, so I removed the milestone tag and will keep it for a later version. Beta 0.7 or even 1.0 would be suitable as this at the moment is nice to have.