Fix non-closable dialog bug

[christianliebel]

• The z-index is now assigned to the menu popup instead of the menu bar itself (this also resolves the FIXME comment)
• Assure the window is within bounds on pointerup

Closes #113

[1j01]

Thanks!

Merged in https://github.com/1j01/jspaint/commit/b051c2b6c98e5580eb71e7f16cff8eb2a0bac444 & https://github.com/1j01/jspaint/commit/d30843a089ad521f0a9e2ab84fecfe78cb44743e (and added a related bit so the close button is always accessible: https://github.com/1j01/jspaint/commit/779eacf7ee68a5ad100eb8b8a1c074e8da3b66c8)