feat: persist fill/fit view mode in window.json (v1.5.0)

The double-click fill/fit mode was hardcoded (image_canvas: _isFilled = true) and
reset on every image change, so it never survived a restart. It is now stored as
"filled" in window.json and restored on launch and on image switch.

WindowPersistence.saveSync() now merges with the existing file, otherwise the
startup/move/resize geometry writes would drop the mode key.

Also: version 1.5.0, CHANGELOG entry, dist/ ships imajviewer_1.5.0_amd64.deb
(replaces 1.4.2).

.gitignore

@@ -49,7 +49,7 @@ app.*.map.json
 *.deb
 *.tar.gz
 *.exe
-!dist/imajviewer_1.4.2_amd64.deb
+!dist/imajviewer_1.5.0_amd64.deb
 !dist/imajviewer_setup_1.1.0.exe
 deb/usr/lib/imajviewer/
 release/