Finalize PotreeDesktop 1.7

[m-schuetz]

• Drag&Drop should automatically load a point cloud if it is a converted point cloud (cloud.js, metadata.json, or parent folder)
• Drag&Drop should present users with a dialog to chose between converter versions 1.7 and 2.0. The dialog should also suggest a default target folder with the option to pick a different one.

[m-schuetz]

The dialog should appear after the drag&drop and only if PotreeDesktop detects las or laz files to be converted.

[m-schuetz]

Also update to new electron version, as suggested by this PR from dependabot: https://github.com/potree/PotreeDesktop/pull/14

[GuillaumeDelorme]

I think you have already done the points of the first 2 comments. For the electron update, it's done in the PR #23