Improve usability of base.path in linkpickers

As documented in [1], you can set a base.path for a linkpicker. However, if you have for one document multiple pickers (image, document, etc), the base.path is most of the time ignored and the picker just opens in the last selected document/folder. This is inconvenient and makes usage feel buggy : The last selected location should be per picker type in a document, not globally

[1] http://www.onehippo.org/library/concepts/document-types/configure-the-linkpicker-to-select-folders-or-filter-on-document-type.html

Acceptance criteria:

• move base.url configuration from a global level to a component level
• backporting to 14.7

Estimate:

• Dev: 2
• QA: .5