Rename all occurances of "accessor" to "(service) provider (plugin)"

- I think the new names are much better and easier to understand
- Rename everywhere: Class names and it's file names, variables,
functions, comments, documentation, path names
- Also rename variables named "info" or "accessorInfo" to "data"
or "providerData"
- Rename "/engine/accessorInfos" to "/engine/serviceProviders",
also rename the installation directory for provider plugins
to serviceProviders
- To have the path in one place it can now be retrieved from
ServiceProvider::installationSubDirectory()
- Retrieve provider plugin file patterns/extensions using
ServiceProvider::filePatterns() / fileExtensions(),
these functions use the mime type to get patterns/extensions
- Add support for the new mime type
"application/x-publictransport-serviceprovider", ie. let the engine
search plugin files with all patterns of the mime type
TODO: Move engine/timetablemate/timetablemate.xml to
engine/serviceproviderplugin.xml
- Rename the log file from "accessors.log" to "serviceproviders.log"
- Rename TimetableAccessor to ServiceProvider and
TimetableAccessorScript to ServiceProviderScript
- Rename TimetableAccessorInfo to ServiceProviderData
- Rename AccessorInfoTester to ServiceProviderDataTester
- Rename AccessorInfoXmlWriter/Reader to
ServiceProviderDataWriter/Reader
- Rename AccessorInfoDialog to ServiceProviderDataDialog
- Rename fields in TimetableMateHelper:
"filenameAccessor" to "filenameProvider" and
"contentsAccessor" to "contentsProvider"
- Update Provider XML format: The root tag is now called
<serviceProvider> instead of <accessorInfo>
- Some provider scripts are also updated to the new script API