X11: implement with_resizable (#540)

[dannyfritz]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created an example program if it would help users understand this functionality

Setting the min/max to the same value creates a non-resizable window with X11.

regarding #540

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FYI, the list of checkboxes got me to go back and edit documentation before creating the PR. ;)

[francesca64]

Thanks! Looks good.

This still shows resize cursors on Xfwm4, but since it's fine on KWin, Mutter, Marco, Compiz, IceWM, Awesome, and Openbox, I'll let you off the hook.

[dannyfritz]

Dang, do you have all those window managers installed locally? I only tested on Mutter with Gnome 3.

[francesca64]

I actually have more than that, though I test WMs in a separate X session from the one I actually use, since some of them like to rearrange all of my windows. Considering most WMs support the --replace flag, it's not as tedious as it sounds.

(The comments in the PR I just linked are a good read if you want to give yourself nightmares. It's quite possible that winit has the most robust X11 window size/position functions of anything in existence, or at least I prefer to believe that.)