Additional features for CSW QGIS plugin

[vesnikos]

• Added ability to get bbox extension from any loaded layer in qgis.
• Added ability to get bbox using geolocation services.
• UI tweaks to accommodate the new features. (UI was updated with Designer)

[tomkralidis]

cc @rduivenvoorde @alexbruy @kalxas

@vesnikos thanks for the PR, these are amazing features! Comments/suggestions:

• filter by layer extent: add an iterm to the "Layer" combobox called "Filter by Layer Extent", which, when selected, does nothing. If I have QGIS open with one layer, I am currently given no choice but to have it selected
• geocoding: I'm confused about the workflow: do you select a placename, hit enter, then search again? When clicking "Search" does this include the placename entered? We should ensure this is clear/fluid (@rduivenvoorde may have suggestions here)
• can we take a stab at documenting these features and the added setting in https://github.com/geopython/MetaSearch/blob/master/docs/howto.rst?
• can we ensure the travis-ci tests pass (see https://travis-ci.org/geopython/MetaSearch/builds/36027089)? The test defs at https://github.com/geopython/MetaSearch/blob/master/.travis.yml do some link checking as well as pep8 checking against all .py files

[kalxas]

@vesnikos will you issue a new PR? I think the review process is going well.

[vesnikos]

@kalxas Yes, I closed the pull request because of some stupid bugs. After I fix them I'll reopen the PL

[vesnikos]

@tomkralidis @kalxas

Additionally from "Select from layer" method implemented a QCompleter to handle multiple locations returned from the geolocation service

[tomkralidis]

FYI note MetaSearch has now been merged into core QGIS, and any pull requests/changes can go through QGIS. Thanks.