Merge branch 'hotfix-1.12.0a17' into devel
Fix: No more modify the selection range during insert source manuiplation since we could hardly backup and restore the original selection ranges.

- if the selection is range(0) -> range(2) -> range(1), the current range becomes range(2) after restore
- if range is selected after-to-before, the current range becomes before-to-after after restore
Use monospace font for insert source.
Adjust the min height of textareas in insert source.
Merge branch 'improve-insert-source' into devel
Fix: old backup .sdf files are not removed.
Slightly code optimization.
Merge branch 'fix-backup' into devel
Remove README and RELEASE files since they are moved to the wiki page.
Update version info.
Merge branch 'release-1.12.0a18'