v0.39.0-beta release

0.39.X contains changes in sync protocol compared to 0.38.X so all instances must be updated.

• sync protocol now has the ability to recover from sync errors - when such error is detected, partial resync is triggered on affected entities
• changes in the app layout:
  • by default is now using frameless window - it is possible to enable the native title bar in Options -> Appearance
  • tabs are now on top, similar to e.g. Chrome and Firefox
• left and right sidebar are now resizable by dragging handles, no need to define the sizing numerically in Options anymore
• printing and HTML export now render more closely to in-editor look #782