Add ability to select an existing project on the Start page.

[1]
Cubic will create a new directory in the user's home directory to store Cubic application information.

    ~/.config/cubic

[2]
Cubic will save project directory paths in a file named:

    ~/.config/cubic/projects.conf

[3]
Five recent projects directory paths will be sorted, with the most recently used listed first.

[4]
The user will be able to select an existing project from a drop down on Cubic's start page, or open a file chooser (existing functionality) to select a project directory.

that's really nice. thanks for all the great work to improve the software

[5]
If the user selects a directory from the drop-down that is no longer valid, and then selects a different valid directory, the invalid directory will be removed from the list of previous projects after the user clicks Next *.

A directory may become invalid if it was deleted or moved, if the user no longer has read/write access to it, or if the directory's file system can no longer be used to customize Linux (i.e. the underlying file system changed to NFTS, FAT, etc. or now points to a remote file system).

This is an easy way for the user to clean-up old non-existing projects from the drop down-list, so invalid projects do not stay in the list forever.

[6]
Changes to the projects list only apply when the user clicks Next *.

Exiting Cubic while on the Start page will not preserve any changes made by the user.

* Note: If the user has selected a Cubic Classic project, then clicking the Migrate button will behave like the Next button.

Fix released in version 2021.10.60.