Improve project create/open interface

[tomcl]

Currently the way you select a project to open, or a directory for a new project, is counter-intuitive. This should be made better.

Implementation notes

Electron file select dialog can be configured to select a directory. That would work with code called after the directory selection that:

1. checks whether selected directory looks like a valid Issie project (dprj file and one or more dgm files)
2. If there are subdirectories makes list of all those with contained Issie projects for user to select from
3. If directory contains no Issie project give error with option to continue dialog box search.

For new projects the dialog should be a screen with two parts (like VS):

1. a box for directory in which to create project with a directory browser based on directory selection as above.
2. a box for the project (and its directory) name.
3. There should be an interlock so that Issie projects cannot be created inside other Issie projects.

[tomcl]

Implementation

electron showOpenDialog should be used for open project and new project. It has properties, given as an array of type OpenDialogOptionsPropertiesArray, which allow create directory, open directory, open file, etc. The right selection of these should work well.