[svn] Added SVN Checkout dialog.

Summary:

Added SVN Checkout dialog with basic and most commonly used SVN Checkout functionality:

• set repo URL;
• change working copy path;
• omit externals.

Additional features:

• automatically pasting repo URL on dialog creation (if its valid);
• automatically forming working copy path based on repo URL (trying to predict repository name);
• user can pick an output folder instead of typing in;
• few error-controls to prevent running 'svn co' with invalid params (disabling 'OK' button).

SVN Checkout can be time consuming.

Reviewers: VDG, Dolphin, meven, elvisangelaccio, ngraham

Reviewed By: Dolphin, meven, elvisangelaccio

Subscribers: anthonyfieroni

Differential Revision: https://phabricator.kde.org/D29489