Fixing project directory creation handling & name edition when custom folder selected

[nebspacefarer]

Hello! First contribution here.

Commit 839b4c4 aim to fix #149 by creating folder only when _create_folder_check is checked on the dialog UI, else it directly emits the signal.

Commit 1fcef2c aim to fix an issue that wrongly reset the content of the _project_path_line_edit.text to the default config projects folder when editing the _project_name_edit.text .
In the commit, if _create_folder_check is checked, it should add a sanitized _project_name_edit.text to the current path after the last parent folder (fetched with the substr() and rfind() present in the commit).
If the _create_folder_check is not checked, we keep the implementation as it is, pulling the config default projects folder and adding the project name.

As writing this, forgot to sanitize the project name, fixing this and this should be ready to merge.

[nebspacefarer]

Added the necessary sanitizing with the existing _format_dir_name() method.
Changing status from draft to ready.

[nebspacefarer]

This last commit was necessary to correctly parse _project_name_edit.text. The added separator allows to retrieve the desired directory by doing a rfind() (reverse find, last occurence) on it.
The separator / should be supported on Windows as well since Vista, but this was only tested on Linux so far.