[svn] Added SVN Checkout dialog.

Summary:
{F8293246}
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