Implement stored searches

[ricardogsilva]

In order make restoring previous searches easier, lets allow the user to save and load search parameters. We already have some of the GUI elements in place, now we need to make them work.

Allow the saving the search parameters in QgsSettings. These shall be saved under the relevant connection key, so that each connection has its own set of stored searches. Additionally, each search shall have a unique id that is never shown to the user and is only used for organizing the settings. A search shall also have a _search_title, which is used as the name of the search for letting the user select it.

Allow restoring saved search parameters too.

# this is a title chosen by the user to refer to the stored search, 
# it is used for showing the stored search in the GUI
qgis_geonode/connections/{connection-uuid}/stored_searches/{search-uuid}/_search_title  

# in this snippet, {search-parameter} is to be replaced with 
# `title`, `abstract`, `category`, etc. This means that there 
# should be a key for each parameter of the search
qgis_geonode/connections/{connection-uuid}/stored_searches/{search-uuid}/{search-parameter}