Refactor X11 Application to make use of the new structure.

[xStrom]

#763 introduced a new instance based Application structure. This PR refactors the X11 Application code to make use of it, as well as adding better error reporting.

The windows map has reentrancy issues. Those were already there and this PR makes no attempt at solving them. I just merely refactored it to the new Application style.

[xStrom]

I also refactored Window to fix the reentrancy issues I was seeing. This means that multiple windows now work on X11! 🥳 I added a button to multiwin to create windows, because X11 doesn't have menus.

Partial invalidation is also really broken on master. Completely black window all the time broken. This made testing my changes here difficult so I ended up fixing that partially as well. The window is no longer black and partial invalidation sort of works.

There's still an issue that the still-valid areas get cleared during partial repaint. I'll see if I can fix it quickly, but otherwise I think that would be fine for a new PR too.

[xStrom]

I ended up fixing partial invalidation too. 🎉

[luleyleo]

The X11 backend works A LOT better with this, thanks!

The inline comments aside I've found two issues:

• Closing one window in the multiwin example closes all windows
• Somtimes widgets disappear, for example hovering over a button in the list example makes the label next to it disappear, or in game_of_life the slider / buttons at the bottom vanish when hovering another one.

[xStrom]

I added more logging and cleanup of windows after destruction. I also solved the partial invalidation issues in game_of_life and others. Those were caused by background painting outside the invalidated area.

The multiwin window closing issue is real but I couldn't find an easy fix. I looked into handling the WM_DELETE_WINDOW event but from what I gathered right now the xcb library we're using doesn't support that event. I'll open a separate issue for that.

[luleyleo]

Looks all good now, thanks 👍