use `getIs...` instead of `is...` to allow these methods to be used as
virtual properties.

This is forconsistency to the rest of the framework.
Only methods that take parameters may be named `is...`.

Doc comments at "yii\helpers\base\FileHelper::findFiles()" have been recomposed using markdown format.

- Added drop/add primary key methods to QueryBuilder.php
- Added mysql specific dropPrimarykey method
- Added sqlite specific dropPrimarykey and addPrimaryKey methods
- Added uint testing for dropPrimarykey and addPrimaryKey methods
- Corrected postgresql column types, by adding length and precision