Window background image

[Vulcalien]

Sharing the progress on this issue: #608

So far, these are the changes I've made:

1. add config settings window_background_type and window_background_image
2. draw a global background image on the window
3. when Terminator.reconfigure is called, also apply config to the windows

The feature 3. was added with the first commit. I don't see any reason for the windows not to apply the config without restarting. This makes it so that the global background image changes without restart, but also all the other settings (fullscreen, always on top...).

Still not ready to merge. Missing features:

1. the preferences menu should have a way to set the global background image
2. settings to choose how the image fills the window/terminal (stretch, scale keeping ratio... (?))

Introduced bugs:

1. When a notebook loses focus, the window background becomes a solid color. Doesn't matter if it was transparent or an image.

Also, are the names window_background_type and window_background_image ok?

[Vulcalien]

Here is a config file to help display what this PR does. Also useful for testing.
Change the paths for settings window_background_image and background_image to whatever image.

config.txt

[mattrose]

If you can solve the introduced bug with the notebook losing focus, that would be huge. I stumbled across this when trying to implement overpaint dimming and I eventually had to pull the code because of it. Thanks so much for all you're doing.

[Vulcalien]

I updated the Window.background_draw function to match the newer and improved version in Terminal. That version, introduced in #713, increased control over how the terminal's background image is drawn. The same now applies to the window's background image.
This implements one of the missing features listed above (2. settings to choose how the image fills...).

The function has practically been copy-pasted from Terminal, so it is a significant code duplication issue. Eventually, if this feature ever gets finished, that should be addressed.